Immunity: The Complete Guide

Key numbers at a glance

How your immune system works

Your immune system is not a single organ but a network of cells, tissues and proteins spread throughout your body. Its job is to tell the difference between what belongs to you and what does not, and then to remove the things that do not: bacteria, viruses, fungi, parasites, and your own cells when they become damaged or abnormal. It does this continuously and mostly silently. The infections you notice are the small fraction of encounters that got far enough to cause symptoms; the vast majority are dealt with before you ever know about them.

The first line of defense is physical. Intact skin is a remarkably effective barrier, and the linings of your nose, airways and gut are coated in mucus that traps germs and moves them away. Tiny hair-like structures in the airways sweep mucus upward, stomach acid destroys many swallowed microbes, and tears and saliva contain enzymes that break down bacteria. This is why a cut, a burn, dry cracked skin or damage to the airways from smoking all make infection more likely: they open a door that is normally shut.

Behind those barriers sit two cooperating systems, usually called innate and adaptive immunity. They work on different timescales and use different tools, and understanding the difference explains a great deal: why you feel ill when you do, why vaccines work, and why some infections can be caught again and again while others give protection that lasts for decades or even for life.

Innate immunity: the fast responders

The innate system reacts within minutes to hours. Its cells recognize broad patterns shared by many germs rather than any single one, and they respond by engulfing invaders, releasing chemical signals and calling in reinforcements. Much of what you experience as being sick, including fever, aches, tiredness, a runny nose and swollen glands, is the innate response at work rather than direct damage from the germ itself.

Adaptive immunity: the specialized memory

The adaptive system is slower to start, typically taking days the first time it meets a new germ, but it is precise. B cells produce antibodies that latch onto one particular target, and T cells either kill infected cells directly or coordinate the rest of the response. Crucially, some of these cells become long-lived memory cells. If the same germ returns, the response is faster and stronger, often fast enough that you never get sick. This memory is the principle behind vaccination, and it is also why viruses that change quickly, such as flu, can infect you repeatedly.

Why "boosting" your immune system is the wrong goal

Supplement packaging and wellness advertising talk about immunity as if it were a volume dial that you should turn up. Immunologists think about it quite differently. A healthy immune system is a balanced one: strong enough to clear infections and abnormal cells, but restrained enough not to attack harmless things or your own tissues. When that balance tips too far toward activity, the result is not better protection but disease.

Allergies are an immune response to something harmless such as pollen, pet dander or a food protein. Autoimmune conditions, including type 1 diabetes, rheumatoid arthritis, lupus, celiac disease and some thyroid disorders, happen when the immune system targets the body itself. Severe infections can also trigger an overwhelming immune reaction that damages organs, which is part of what makes sepsis so dangerous. None of these would be improved by a product that genuinely made the immune system more active, which is a good reason to be skeptical of any product that claims to do so.

A more accurate goal is to give your immune system what it needs to function normally and to remove things that get in its way. In practice that means getting the vaccines recommended for you, sleeping enough, being physically active, eating a varied diet that covers your nutrient needs, not smoking, keeping alcohol low, and managing long-term conditions such as diabetes well. Unglamorous, but each has real evidence behind it.

What weakens your defenses

Some influences on immunity are outside your control; others are habits or conditions that can be changed or treated. Either way, knowing your own risk factors tells you when to act early on an infection rather than waiting to see how it goes. If one applies to you, ask your doctor in advance exactly what to do when you get sick.

Age matters at both ends of life. Babies have immature immune systems and rely partly on antibodies passed on during pregnancy and through breast milk, which is why fever in a very young baby is treated so seriously. In older adults the immune system gradually becomes less responsive, so infections can be more severe and vaccines may produce a weaker response, which is why some come in higher-dose or enhanced versions for older people. Fever may also be less obvious, so other changes matter more.

Many medical conditions and treatments affect immune function. These include diabetes, particularly when blood sugar is poorly controlled, chronic kidney and liver disease, HIV, some cancers and their treatment, and conditions that require medicines which deliberately damp down the immune system, such as those used after organ transplants or for some autoimmune diseases. Long courses of steroid tablets have a similar effect. If any of these apply to you, your own care team will usually give you specific advice about infections and vaccines, and it should take priority over general guidance.

• Smoking and vaping damage the airway lining and its mucus-clearing system, making respiratory infections more likely and more severe
• Regularly sleeping less than 7 hours is linked to a higher chance of catching colds and a weaker antibody response to vaccines
• Heavy drinking impairs immune cells and raises the risk of pneumonia and other serious infections
• Long-term stress, through stress hormones such as cortisol, can suppress parts of the immune response
• Poor nutrition, including specific deficiencies such as vitamin D, zinc or protein, impairs immune function

Vaccination: the most specific protection there is

A vaccine shows your adaptive immune system a harmless version or piece of a germ, so that it can build memory without you having to survive the disease first. When the real infection arrives, your body recognizes it and responds quickly. No food, supplement or lifestyle change produces this kind of targeted, durable protection against a named infection, which is why vaccination sits at the top of every public health recommendation for preventing infectious disease.

The CDC publishes immunization schedules for children and teens and for adults, and they are updated as evidence and vaccines change. The flu vaccine is recommended every season for most people aged 6 months and older, because flu viruses change from year to year and protection from last season fades. Adults aged 50 and older are recommended to get 2 doses of the recombinant shingles vaccine. Pneumococcal and RSV vaccines are recommended for older adults and for some younger people with certain health conditions, and because the age guidance for these has changed in recent years, it is worth asking your doctor or pharmacist what currently applies to you. COVID-19 vaccine recommendations have also changed several times, so check the current advice for your age and health.

Adults also need a tetanus and diphtheria booster every 10 years, with one dose given as Tdap, which adds protection against whooping cough. Many adults have gaps from childhood: catch-up doses of MMR, chickenpox, hepatitis B and HPV vaccines can all be given in adulthood, depending on age and history. Travel can also call for extra vaccines, some of which need several weeks of lead time, so plan a visit well before you go.

Most vaccine side effects are mild and short-lived: a sore arm, tiredness or a low fever for a day or two. Severe allergic reactions are rare, and vaccinators are trained and equipped to treat them on the spot if one happens. Tell the person vaccinating you about previous reactions, allergies, pregnancy, a weakened immune system or a bleeding disorder beforehand.

Cost and where to get vaccinated

Under the Affordable Care Act, most private health plans must cover routinely recommended vaccines with no copay when you use an in-network provider. Medicare Part B covers flu, pneumococcal, hepatitis B for people at increased risk and COVID-19 vaccines, and Medicare Part D plans cover other recommended adult vaccines, including shingles, with no out-of-pocket cost. Medicaid also covers recommended vaccines. Children whose families cannot afford vaccines may be eligible for the federal Vaccines for Children program. Many pharmacies give a wide range of adult vaccines without an appointment, which makes staying up to date easier than many people assume.

Sleep, activity, stress and alcohol

Sleep has one of the clearest links to infection of any everyday habit. During sleep the body releases signaling molecules that help coordinate immune responses, and memory cells consolidate what they have learned. Short sleepers are more likely to become ill when exposed to cold viruses, and people short of sleep around a vaccination tend to produce fewer antibodies. The CDC recommends that adults get 7 or more hours a night; teenagers and children need more. If you are consistently short of that, improving sleep is likely to do more for your defenses than anything you can buy.

Regular moderate physical activity supports immune function, along with almost every other aspect of health. US guidelines recommend at least 150 minutes a week of moderate-intensity activity, such as brisk walking, plus muscle-strengthening activities on at least 2 days a week. Active people tend to have fewer and milder respiratory infections, and exercise improves blood sugar control, sleep and mood, all of which feed back into immunity. Very prolonged, intense endurance efforts may temporarily lower some defenses, but that is not a concern for the overwhelming majority of people.

Chronic stress raises cortisol and other hormones that, over time, can suppress parts of the immune response and disrupt sleep. Regular activity, social connection, time outdoors and talking therapies such as cognitive behavioral therapy all help when stress is persistent. If stress has tipped into feeling hopeless or unable to cope, talk to your doctor, and if you are in crisis call or text 988.

Alcohol interferes with immune cells in the airways and the gut, and heavy drinking is a recognized risk factor for pneumonia and other serious infections. The Dietary Guidelines for Americans advise that adults who drink should limit intake to 2 drinks or fewer a day for men and 1 or fewer for women, and that drinking less is better for health than drinking more. Smoking is even more directly harmful to respiratory defenses; quitting is one of the most effective things a smoker can do for their lungs and their immune system, and free help is available by calling 1-800-QUIT-NOW.

Food, nutrients and supplements

Immune cells are among the most active cells in the body and need a steady supply of energy, protein, vitamins and minerals. Genuine deficiency of nutrients such as vitamin D, zinc, vitamin A, vitamin C, iron or protein impairs immune function, and correcting a deficiency helps. The mistake is assuming that if a little is good, a lot must be better. Once your needs are met, extra amounts do not make your immune system work harder, and some nutrients cause harm at high doses.

A varied diet built on vegetables, fruit, whole grains, beans, nuts, dairy or fortified alternatives and lean protein foods covers most needs. Supplements are useful for a diagnosed deficiency, a diet that excludes a whole food group, or a life stage that raises needs. In the US, supplements do not need FDA approval for safety or effectiveness before they are sold, so label claims guarantee nothing.

Vitamin D

Vitamin D is involved in both innate and adaptive immunity, and low levels are common, particularly in people with darker skin, those who spend little time outdoors, older adults and people with obesity. The RDA is 15 mcg (600 IU) a day for adults 19 to 70 and 20 mcg (800 IU) for adults over 70, and the upper limit for adults is 100 mcg (4,000 IU) a day. Few foods are naturally rich in it; fatty fish, egg yolks and fortified milk, plant milks and cereals are the main sources. A modest supplement makes sense for many people, but high doses taken without testing or medical advice can raise calcium levels and cause harm.

Vitamin C

Vitamin C supports the function of several types of immune cell and is needed to make collagen for skin and wound healing. The RDA is 90 mg a day for adult men and 75 mg for adult women, with an extra 35 mg for people who smoke, and the upper limit is 2,000 mg. A single orange or a serving of bell pepper covers most of a day. For the general population, regular vitamin C supplements do not reliably prevent colds, and trials suggest at most a small reduction in how long a cold lasts. Doses above the upper limit can cause diarrhea and nausea, and people with a history of kidney stones should be cautious.

Zinc

Zinc deficiency genuinely impairs immunity, and zinc is needed for wound healing and taste. The RDA is 11 mg a day for adult men and 8 mg for adult women, with an upper limit of 40 mg from all sources combined, including multivitamins and cold lozenges. Oysters, meat, poultry, seafood, beans, nuts, whole grains and fortified cereals all supply it, though zinc from plant foods is absorbed less well. Long-term high-dose zinc can cause copper deficiency, and zinc nasal sprays have been linked to permanent loss of smell, so avoid them.

Gut health and probiotics

A large share of the body's immune tissue sits in the gut wall, and the microbes living in the gut help train and regulate it. A variety of fiber-rich plant foods is the best-supported way to support them. The Dietary Guidelines recommend about 14 g of fiber per 1,000 calories, roughly 22 to 34 g a day for most adults, and most Americans eat well below that. Probiotic supplements are another matter: effects depend on the specific strain, and products are not interchangeable.

Colds, flu and fever

Colds and flu are both viral infections of the airways, they circulate at the same time of year, and they are often confused. The most practical difference is how quickly symptoms start. A cold creeps up over a day or two, usually beginning with a scratchy throat and a runny or blocked nose, and fever is uncommon in adults. Flu tends to hit abruptly, over a matter of hours, with fever, chills, body aches, exhaustion and a dry cough. A typical cold lasts about 7 to 10 days; flu usually improves within a week or two, though tiredness can linger.

The distinction matters because flu has both a vaccine and antiviral medicines, and some people are at much higher risk of complications such as pneumonia. Prescription flu antivirals work best when started within 2 days of symptoms beginning, so if you are 65 or older, pregnant, have a long-term condition such as asthma, heart disease or diabetes, or have a weakened immune system, contact your doctor early rather than waiting to see whether you get worse. Testing can also distinguish flu from COVID-19 and RSV, which matters for treatment decisions in higher-risk people.

For most healthy people, both illnesses clear on their own with rest, fluids and time. Acetaminophen (Tylenol) or ibuprofen can ease aches and fever. Check labels if you take more than one cold product, because many contain acetaminophen and doubling up is a common route to accidental overdose. Never give aspirin to children or teenagers with a viral illness, because of the risk of Reye's syndrome, and do not give cough and cold medicines to young children unless their doctor advises it.

Understanding fever

A fever is commonly defined as a temperature of 100.4°F (38°C) or higher. It is part of the immune response rather than a disease in itself: a raised temperature makes the body a less comfortable place for many germs and speeds up some immune processes. For that reason, the aim of treatment is comfort rather than bringing the number back to normal. Keeping fluids up matters more, because dehydration is the main practical risk in most fevers. Dress lightly rather than bundling up, and avoid sponging with cold water, which causes shivering.

How a person looks and behaves tells you more than the reading on the thermometer. A child with a fever who is playing between doses of medicine is reassuring; a child with a modest temperature who is limp, unusually drowsy or hard to wake is not. The exception is young babies. Any fever of 100.4°F (38°C) or higher in a baby under 3 months needs a call to the doctor right away, or a trip to the emergency room, whatever else is going on. Ibuprofen should not be given to babies under 6 months unless a doctor advises it.

When to stay home

Current CDC advice for respiratory viruses, including flu, COVID-19 and RSV, is to stay home and away from others until, for at least 24 hours, your symptoms are getting better overall and you have had no fever without using fever-reducing medicine. For the following 5 days, take extra care: wear a well-fitting mask around others, keep your distance, wash your hands often and improve ventilation, particularly around people at higher risk.

Antibiotics and why they do not work on viruses

Antibiotics kill bacteria or stop them multiplying by attacking structures that bacteria have and human cells do not, such as bacterial cell walls and the machinery bacteria use to make proteins and copy their DNA. Viruses have none of those structures, and they reproduce inside your own cells, so antibiotics have no effect on them at all. Colds, flu, COVID-19, most sore throats, most sinus infections and most cases of acute bronchitis are viral. Taking antibiotics for them gives you the side effects, such as diarrhea, rashes, yeast infections and occasionally serious allergic reactions, without any of the benefit.

Every course of antibiotics also gives the bacteria living in and on you a chance to evolve resistance. Resistant bacteria can then cause infections that are hard or occasionally impossible to treat. The CDC estimates that antimicrobial-resistant infections affect more than 2.8 million people in the US each year and cause more than 35,000 deaths. Antibiotics also disrupt the gut microbiome, usually temporarily, and they are a major risk factor for C. diff, a bacterial infection that causes severe diarrhea.

Some common beliefs push people toward unnecessary antibiotics. Green or yellow mucus is not a reliable sign of a bacterial infection; the color comes from immune cells and appears in viral infections too. An illness that lasts longer than you expected is not necessarily bacterial either, as a cough after a cold often lingers for 2 to 3 weeks. When the picture is uncertain, your doctor may suggest watchful waiting, or a delayed prescription that you fill only if you are not improving by a certain point.

• Take antibiotics exactly as prescribed, and ask whether you should finish the course or stop when told to
• Never take leftover antibiotics or someone else's
• Dispose of unused medicines at a drug take-back site or as the FDA advises
• Ask your doctor which symptoms should make you come back, and by when

Stopping infections from spreading

Handwashing remains one of the cheapest and most effective infection-control measures there is. Germs from noses, mouths and the bathroom travel on hands to door handles, phones, food and other people. The CDC recommends washing with soap and clean running water for at least 20 seconds, covering the backs of the hands, between the fingers, the thumbs and under the nails, then drying them properly, since wet hands transfer germs more easily. Plain soap works; antibacterial soap offers no extra benefit for everyday use. The key moments are before eating or preparing food, after using the bathroom or changing a diaper, after blowing your nose or coughing, and before and after caring for someone who is sick.

When soap and water are not available, an alcohol-based hand sanitizer containing at least 60 percent alcohol is a reasonable substitute, but it has limits. It works poorly on visibly dirty or greasy hands, and it does not work well against norovirus, the highly contagious stomach bug behind many outbreaks on cruise ships, in schools and in nursing homes. During any stomach illness, use soap and water. Anyone who has had norovirus should not prepare food for others for at least 2 days after their symptoms stop, because they can still shed the virus.

For respiratory viruses, fresh air is an underused tool. Viruses spread in tiny particles that build up in poorly ventilated rooms, so opening windows or doors, running fans to move air outside, or using a portable HEPA air cleaner all help reduce the amount in the air, even if only for a few minutes at a time. A well-fitted NIOSH-approved N95 respirator gives the most protection of any mask type. Covering coughs and sneezes with a tissue or your elbow, and staying home while you have a fever, protect the people around you.

Caring for someone sick at home

Where you can, keep the sick person in one room and one bathroom, and do not share towels, cups or utensils. Clean frequently touched surfaces, then disinfect where needed; after vomiting or diarrhea, use a bleach-based cleaner or an EPA-registered product effective against norovirus. Wash soiled laundry on the longest cycle and machine dry it, and look after your own sleep, since caregivers are often next.

Pregnancy, children, older adults and weakened immune systems

Pregnancy changes the immune system in ways that protect the baby but make some infections, including flu and COVID-19, more severe for the pregnant person. The flu vaccine is recommended during pregnancy, in any trimester. A Tdap vaccine is recommended during each pregnancy, ideally early in the third trimester, so that antibodies pass to the baby and protect them against whooping cough before they can be vaccinated themselves. RSV protection for newborns can come either from a vaccine given late in pregnancy during RSV season or from an antibody shot given to the baby; your obstetric provider can explain which applies. Live vaccines such as MMR and chickenpox are not given during pregnancy, so check your immunity before you conceive if you can.

Children catch many infections in their first years of child care or school, because their immune systems are meeting common viruses for the first time, and frequent colds can be entirely normal. Following the CDC childhood immunization schedule with your child's doctor, including catch-up doses, is the most effective step you can take. Breastfeeding, where possible, passes on antibodies. Teach handwashing early, and keep children home while they have a fever.

Older adults face more severe infections and may not show the classic signs. Fever can be absent or low, and the first sign of a urinary tract infection, pneumonia or sepsis can be new confusion, a fall, or simply being much less able to manage than usual. For that reason, vaccines including flu, shingles, pneumococcal and RSV matter more with age, and changes in an older person's behavior or alertness during an illness deserve prompt medical attention.

People with weakened immune systems, whether from a condition such as HIV or from treatment such as chemotherapy, transplant medicines, long-term steroids or some biologic drugs for autoimmune disease, need individualized advice. Some vaccines are recommended more urgently, some live vaccines must be avoided, and fever can be an emergency rather than a nuisance; people on chemotherapy are usually told to call their care team right away for a fever. Follow the instructions of your own team, which will be tailored to your treatment and override general advice.

When infections may point to an immune problem

Everyone gets infections, and catching several colds a year is not a sign that something is wrong with your immune system. A pattern that is out of proportion is different. Doctors take notice when infections are unusually frequent, unusually severe, caused by unusual germs, slow to clear, or keep needing antibiotics. Repeated pneumonia, deep skin abscesses, persistent thrush in an adult, or infections that require intravenous antibiotics are examples. Weight loss, night sweats or persistently swollen glands alongside infections also need investigating.

Causes include undiagnosed diabetes, HIV, the effects of a medicine, and inherited immune disorders, sometimes called primary immunodeficiencies, which are uncommon and can be diagnosed in adulthood. Testing usually starts with simple blood tests and may lead to referral to an immunologist. The CDC recommends that everyone aged 13 to 64 be tested for HIV at least once as part of routine care, and more often for people at ongoing risk.

Talking to your doctor

Arrive with specifics: the infections you have had over the past year or two, how long each lasted, what treatment you needed, every medicine and supplement you take, and your vaccination record, which a pharmacy or your state immunization registry can supply. Useful questions include whether your vaccines are up to date, whether you should be tested for a deficiency before taking a supplement, how fast to call about flu symptoms, and which signs mean you should be seen the same day. If a condition or treatment affects your immunity, ask for a written fever plan.

A practical yearly plan

Immunity is not built in a week, and the habits that matter most are the same ones that support heart, brain and metabolic health. The plan below spreads the useful actions through the year. Early fall is the most important checkpoint: flu and other respiratory viruses usually start circulating in the US in October and peak over the winter, so aim for your flu vaccine in September or October. A later shot is still worthwhile, because flu often continues into spring.

• January: review last year, including how many infections you had, and book a yearly checkup if you are due
• February to April: set a consistent sleep schedule that gives you at least 7 hours, and build toward 150 minutes a week of activity
• May: check your vaccination record against the CDC adult schedule and note any gaps, including shingles, Tdap and catch-up doses
• June to August: plan any travel vaccines well ahead, and review supplements with your doctor or pharmacist
• September to October: get your flu vaccine and any other recommended respiratory vaccines
• November: restock your medicine cabinet with a thermometer, acetaminophen or ibuprofen, and oral rehydration solution, and check expiration dates
• December: during the holidays and peak virus season, stay home when sick and improve ventilation when gathering indoors

Every guide in this topic

• The Evidence on Immune Function, Without the Hype
• Vaccination: What the Evidence Actually Says
• Colds and Flu: What Published Guidance Supports
• The Evidence on Antibiotics, Without the Hype
• What the Guidance Really Says About Hand Hygiene
• The Evidence on Fever, Without the Hype
• Vitamin C: What the Evidence Actually Says
• Zinc: What the Evidence Actually Says
• Gut Health and Immunity: What the Evidence Actually Says
• The Evidence on Infection Prevention at Home, Without the Hype

Frequently asked questions

Can I boost my immune system with supplements?

Supplements help when you are deficient in a nutrient, such as vitamin D or zinc, and correcting a deficiency improves immune function. They do not push a well-nourished immune system above normal, and some cause harm at high doses. Vaccines, sleep, activity and not smoking have far stronger evidence.

Does vitamin C prevent colds?

For most people, no. Taking vitamin C regularly does not reliably reduce the chance of catching a cold, though trials suggest it may slightly shorten how long a cold lasts. Starting it after symptoms begin shows little benefit. The RDA of 90 mg for men and 75 mg for women is easy to get from fruit and vegetables, and doses above 2,000 mg a day can cause diarrhea and nausea.

Why do I need a flu shot every year?

Flu viruses change from season to season, so the vaccine is updated each year to match the strains expected to circulate. Protection from a flu shot also fades over the months after you get it. Getting vaccinated in September or October covers the winter peak, but a later shot is still worthwhile.

Can the flu shot give me the flu?

No. Flu shots contain either inactivated virus or a single protein from the virus, neither of which can cause flu. Some people feel achy or tired, or have a low fever, for a day or two afterward, which reflects the immune system responding. Because the shot takes about 2 weeks to build protection, people sometimes catch flu or another virus in that window and blame the vaccine.

Should I ask for antibiotics for a bad cold or green mucus?

Usually not. Colds, flu and most sore throats, sinus infections and coughs are caused by viruses, which antibiotics cannot treat. Green or yellow mucus is not a reliable sign of bacterial infection. See your doctor if symptoms last more than about 10 days, get worse after improving, or include high fever or trouble breathing.

Is hand sanitizer as good as washing my hands?

Not always. Soap and water for at least 20 seconds is best, especially when hands are visibly dirty or greasy and during stomach bugs, because sanitizer does not work well against norovirus. When soap and water are not available, an alcohol-based sanitizer with at least 60 percent alcohol is a good second choice for most respiratory germs.

Should I bring a fever down with medicine?

Only if it makes the person more comfortable. Fever is part of the immune response, and lowering the number does not speed recovery. Acetaminophen (Tylenol) or ibuprofen can ease aches and help someone rest and drink. Never give aspirin to children or teenagers, and do not give ibuprofen to babies under 6 months without a doctor's advice. A fever of 100.4°F (38°C) in a baby under 3 months needs urgent medical advice.

Do probiotics help my immune system?

The evidence is mixed and depends on the specific strain, so one probiotic product cannot be assumed to work like another. Eating plenty of fiber-rich plant foods is the best-supported way to support healthy gut microbes. People with weakened immune systems should check with their care team before taking live probiotic supplements, as rare serious infections have been reported.

How many colds a year is normal?

Adults commonly catch around two or three colds a year, and young children often catch many more, especially in their first years of child care or school. That alone is not a sign of a weak immune system. See your doctor if infections are unusually severe, keep needing antibiotics or hospital care, are caused by unusual germs, or come with weight loss, night sweats or persistently swollen glands.

When should I go back to work or school after being sick?

For respiratory viruses such as flu, COVID-19 and RSV, the CDC advises staying home until, for at least 24 hours, your symptoms are improving overall and you have had no fever without fever-reducing medicine. For the next 5 days, wear a mask around others and take extra care with hygiene. After norovirus, avoid preparing food for others for at least 2 days after symptoms stop.

When to speak to a doctor

See your doctor for infections that are unusually frequent, severe or slow to clear, that keep needing antibiotics, or that come with unexplained weight loss, night sweats or persistently swollen glands. Contact your doctor early if you are 65 or older, pregnant, have a long-term condition or a weakened immune system and develop flu symptoms, since antivirals work best within 2 days. Call the doctor right away for any fever of 100.4°F (38°C) or higher in a baby under 3 months, and call your care team at once for a fever during chemotherapy. Call 911 for difficulty breathing, chest pain, new confusion, a seizure, a stiff neck with fever, a rash that does not fade under pressure, signs of sepsis such as a racing heart with clammy skin and extreme illness, or swelling of the face or throat after a vaccine or medicine. If someone has taken too much of a medicine, call Poison Control at 1-800-222-1222.

Where this comes from

• National Institute of Allergy and Infectious Diseases — Immune System
• CDC — Adult Immunization Schedule by Age
• CDC — Child and Adolescent Immunization Schedule by Age
• CDC — Preventing Spread of Respiratory Viruses When You're Sick
• CDC — Treating Flu with Antiviral Drugs
• CDC — Antibiotic Resistance Threats in the United States, 2019
• CDC — When and How to Wash Your Hands
• NIH Office of Dietary Supplements — Dietary Supplements for Immune Function and Infectious Diseases
• NIH Office of Dietary Supplements — Vitamin D: Fact Sheet for Consumers
• American Academy of Pediatrics — Fever and Your Baby

How this page was put together

Every figure, recommendation and warning above is taken from the guidance named in Where this comes from. Nothing on this page is inferred, summarized or generalized beyond what those bodies publish: where they give a number, it is their number, and where they are silent, so are we. Pages are rebuilt whenever the underlying record changes. Sign-off by a named clinician, where it has happened, is shown at the top of the page along with the date.

Medical disclaimer

This article is general information, not medical advice. It cannot account for your own history, medication or circumstances. Always speak to your doctor, pharmacist or another qualified health professional before making changes that affect your treatment, and never delay seeking medical advice because of something you have read here. In an emergency, call 911.