Infectious Diseases: The Complete Guide

A practical guide to how common infections spread in the US and how to prevent, recognize and manage them, from colds, flu, COVID-19 and RSV to stomach bugs, strep throat, measles, tick- and mosquito-borne illness, hepatitis A and tuberculosis. It is written for adults looking after themselves, their children or older relatives.

Key numbers at a glance

Germs, infections and how your body fights back

An infection happens when a germ, such as a virus, bacterium, fungus or parasite, gets into the body and multiplies. Many infections are mild and clear up on their own as the immune system responds; others can cause serious illness, especially in babies, older adults, pregnant people and anyone with a weakened immune system or a chronic condition. The symptoms you feel, such as fever, aches, a runny nose or tiredness, are often signs of your immune system at work rather than direct damage from the germ itself.

The type of germ matters because it decides which treatments work. Most coughs, colds, sore throats and stomach bugs are caused by viruses, which antibiotics cannot treat. Bacterial infections, such as strep throat, Lyme disease and tuberculosis, usually do need antibiotics. A few viral infections, including flu and COVID-19, have antiviral medicines that work best when started early. Knowing which is which explains why your doctor may test before prescribing, and why the advice for one illness can be the opposite of the advice for another.

Your body remembers many germs it has met before. After an infection or a vaccine, the immune system keeps memory cells that can respond faster the next time. That is the basis of vaccination, and it is also why some illnesses, such as measles or hepatitis A, usually happen only once, while others, such as colds and flu, return because the viruses change or because there are many different types.

How infections spread

Germs travel by a handful of routes, and each route has its own best defense. Understanding them helps you match the precaution to the illness instead of guessing. Many illnesses spread in more than one way, so a combination of measures usually works best. For example, fresh air and masks help most against respiratory viruses, soap and water against norovirus, careful cooking against foodborne germs, and repellent against ticks and mosquitoes.

Timing matters as much as the route. Many infections spread before symptoms start or before people realize they are sick, which is why habits such as hand washing and covering coughs are worth practicing all the time, not only when someone looks ill. People also often stay contagious for a while after they start to feel better, which is why guidance on returning to work, school or food handling is tied to specific milestones such as being fever-free or having taken antibiotics for a set time.

• Through the air: respiratory viruses such as flu, COVID-19 and RSV spread through droplets and smaller particles when people breathe, talk, cough or sneeze. Measles and tuberculosis can stay in the air of a room for a long time.
• Through hands and surfaces: germs on hands, doorknobs, phones and toys reach the eyes, nose and mouth. Norovirus is especially good at surviving on surfaces.
• Through food and water: tiny amounts of stool, or contaminated food and water, spread norovirus, hepatitis A and many foodborne illnesses.
• Through bites: blacklegged (deer) ticks spread Lyme disease, and mosquitoes spread West Nile virus and other infections.
• Through blood and sexual contact: hepatitis B and C, HIV and sexually transmitted infections spread this way and are covered in more detail in sexual health guides.

Everyday habits that prevent infection

Clean hands are one of the most effective ways to prevent many infections. Wash with soap and water for at least 20 seconds, scrubbing the backs of hands, between fingers and under nails, especially after using the toilet, changing diapers, caring for someone who is sick and before eating or preparing food. Alcohol-based hand sanitizer is a useful backup when soap and water are not available, but it does not work well against norovirus, and it does not remove visible dirt, so soap and water remain the first choice when you can get to a sink.

Cover coughs and sneezes with a tissue or your elbow, then wash your hands. Stay home when you are sick, and keep sick children home from child care and school, so you do not pass illness on to people at higher risk. Cleaner indoor air also lowers the spread of respiratory viruses: open windows when weather allows, run heating and cooling system fans, and consider a portable HEPA air cleaner in a room where someone is sick. Masks can add protection during respiratory virus season for people at higher risk and for anyone who is sick and must be around others.

Food safety prevents a large share of stomach illness. Clean hands and surfaces, separate raw meat from ready-to-eat food, cook foods to safe temperatures and chill leftovers promptly. Do not prepare food for others while you have vomiting or diarrhea. For ticks and mosquitoes, prevention focuses on repellents, protective clothing and removing places where mosquitoes breed, covered later in this guide.

Vaccines: your strongest protection

Vaccines train the immune system to recognize a germ without having to survive the illness first. They have made diseases such as measles, polio and diphtheria rare in the US, but those diseases have not disappeared, and outbreaks happen where groups of people are not vaccinated. Vaccines also protect people around you who cannot be vaccinated or who respond less well, such as young babies and people having cancer treatment.

Several vaccines are directly relevant to the illnesses in this guide. The flu vaccine is recommended every year for everyone 6 months and older, ideally by the end of October, because flu viruses change and protection fades. Two doses of MMR vaccine, at 12 to 15 months and 4 to 6 years, give about 97% protection against measles, and adults born in 1957 or later without evidence of immunity need at least 1 dose. Hepatitis A vaccine is given as 2 doses starting at 12 to 23 months, and is recommended for travelers to many countries and adults at higher risk. RSV protection is available for babies, through a vaccine in pregnancy or an antibody shot for the infant, and an RSV vaccine is recommended as a single dose for everyone 75 and older and for adults 50 to 74 at increased risk.

COVID-19 vaccines are updated each season, and recommendations about who should get them have changed in recent years, so ask your doctor or pharmacist what is recommended for you. Recommendations for other vaccines can also be updated, and the best way to stay current is to review your family's vaccine records at yearly checkups. Many pharmacies give adult vaccines, and most health plans cover recommended vaccines with no copay.

Common side effects, such as a sore arm, mild fever or tiredness for a day or two, are signs that the immune system is responding. Serious allergic reactions are rare, and vaccination sites are prepared to treat them, which is why you may be asked to wait for a short time after a shot. Tell the person giving the vaccine about any past severe reaction, a weakened immune system or a pregnancy, since some vaccines, particularly live vaccines such as MMR and chickenpox, are not given in those situations.

Respiratory illnesses: colds, flu, COVID-19 and RSV

Respiratory viruses peak in the fall and winter in most of the US and cause most of the illness people catch each year. Many of them start with similar symptoms, so it is often impossible to tell which virus you have without a test. The good news is that the CDC now gives the same basic advice for all of them: stay home and away from others until your symptoms are improving overall and you have had no fever for 24 hours without fever-reducing medicine. For the next 5 days, take extra precautions, such as wearing a mask, keeping your distance, washing your hands and improving indoor air, since you may still be able to spread the virus.

A cold or the flu?

Colds usually start gradually with a runny or stuffy nose, sneezing and a sore throat, and most people keep functioning. Flu tends to start suddenly with fever, chills, body aches, headache and exhaustion, and often puts people in bed for several days. Flu can lead to pneumonia and hospitalization, particularly in people 65 and older, young children, pregnant people and those with asthma, diabetes, heart disease or other chronic conditions. Antiviral medicines for flu work best when started within 48 hours of symptoms, so if you are in a higher-risk group, call your doctor early in a flu-like illness rather than waiting to see if it gets worse.

COVID-19

COVID-19, caused by the SARS-CoV-2 virus, ranges from a mild cold-like illness to severe pneumonia. It still causes many hospital stays each year, mostly among older adults and people with chronic conditions or weakened immune systems. Keep a few FDA-authorized home tests on hand and check their expiration dates. If a home antigen test is negative but you have symptoms, test again 48 hours later, because early tests can miss an infection. Antiviral treatment is available for people at higher risk and works best started within 5 days of symptoms, so if you are 65 or older or have a chronic condition, contact your doctor, a pharmacy or a telehealth service quickly after a positive test.

RSV

Respiratory syncytial virus (RSV) usually causes cold-like symptoms that improve within 1 to 2 weeks, but it is the leading cause of hospitalization in US infants and can cause serious lung infections in older adults. In babies, watch for fast or labored breathing, poor feeding and fewer wet diapers. For a congested baby, saline drops and gentle suction can make feeding easier. Keep people with colds away from young babies, and ask everyone to wash their hands before holding them.

Antibiotics, antivirals and caring for yourself at home

Antibiotics kill bacteria or stop them from growing. They do nothing against viruses, so they will not help a cold, flu, COVID-19, RSV or most sore throats and coughs. Taking them when they are not needed causes side effects, such as diarrhea, rashes and allergic reactions, and adds to antibiotic resistance, which makes future bacterial infections harder to treat. If you are prescribed an antibiotic, take every dose exactly as directed, for as long as prescribed, even after you feel better. Never use leftover antibiotics or someone else's prescription.

Antiviral medicines are different. For flu and COVID-19, they can shorten illness and lower the risk of complications in people at higher risk, but only if started early, within about 48 hours for flu and 5 days for COVID-19. That is why calling your doctor promptly matters if you are 65 or older, pregnant, or have a chronic condition or weakened immune system.

For most viral illnesses, home care is what helps. Rest, drink plenty of fluids and use acetaminophen (Tylenol) or ibuprofen for fever and aches as the label directs, watching for acetaminophen in combination cold products so you do not double up. Do not give aspirin to children or teens with flu-like illness or chickenpox, because of the risk of Reye's syndrome. Over-the-counter cough and cold medicines should not be given to children under 4 without a doctor's advice. Honey can soothe a cough in children over 1 year old, but never give honey to a baby under 1.

Stomach bugs, norovirus and hepatitis A

Norovirus, often called the "stomach bug," is the leading cause of vomiting, diarrhea and foodborne illness in the US. It is extremely contagious: it spreads through food, surfaces and close contact, and only a tiny amount of virus is needed to make someone sick. Symptoms usually start 12 to 48 hours after exposure with sudden vomiting, diarrhea and stomach cramps, and most people feel better in 1 to 3 days. Outbreaks are common in households, child care centers, schools, nursing homes and cruise ships.

The main danger is dehydration, especially in babies, young children, older adults and people with chronic conditions. Sip water, oral rehydration solution or broth often, in small amounts if vomiting is frequent. Oral rehydration solutions replace salts as well as water and are a better choice than sports drinks for young children. Signs of dehydration include passing very little urine, a dry mouth, dizziness when standing and, in children, crying without tears or unusual sleepiness.

Stopping the spread takes more than hand sanitizer. Wash hands with soap and water, wear disposable gloves to clean up vomit or diarrhea, and disinfect the area with a bleach solution or an EPA-registered disinfectant labeled for norovirus. Wash soiled laundry on the longest hot cycle and machine dry it. Do not prepare food for others while you are sick and for at least 2 days after symptoms stop.

Hepatitis A

Hepatitis A is a liver infection spread when tiny amounts of stool from an infected person get into food, water or onto hands. Symptoms appear about 15 to 50 days after exposure and can include fatigue, nausea, belly pain, dark urine, pale stools and yellowing of the skin or eyes. It can cause weeks of illness and, rarely, liver failure, but it does not become a chronic infection and most people recover fully. If you have been exposed, call your doctor or health department quickly: a vaccine or immune globulin given within 2 weeks can help prevent illness. While your liver is inflamed, avoid alcohol and do not take extra acetaminophen unless your doctor agrees.

Sore throats and strep throat

Most sore throats are caused by viruses and get better on their own within about a week. Strep throat is the important exception. It is caused by group A Streptococcus bacteria, is most common in children aged 5 to 15, and needs antibiotics, which shorten the illness, reduce spread and prevent rare but serious complications such as rheumatic fever, which can damage the heart.

Strep typically causes a sore throat that starts quickly, pain when swallowing, fever, red and swollen tonsils sometimes with white patches, and swollen glands in the neck. Children may also have a headache, stomach pain or vomiting. A cough, runny nose, hoarseness and pink eye point toward a virus instead. Because the symptoms overlap, a rapid strep test or throat culture is needed to confirm the diagnosis; doctors do not usually prescribe antibiotics for a sore throat without one.

With antibiotics, most people feel better within a day or two. Children and adults can usually go back to school or work once the fever is gone and they have taken antibiotics for at least 12 hours. Use acetaminophen or ibuprofen for pain, offer cool drinks and soft foods, replace toothbrushes, and do not share cups or utensils. See your doctor again if you do not improve after 48 hours of antibiotics.

Measles and other vaccine-preventable outbreaks

Measles is one of the most contagious diseases known. Up to 9 in 10 unprotected people who are close to an infected person will catch it, and the virus can linger in the air for up to 2 hours after the sick person has left a room. People are contagious from 4 days before to 4 days after the rash appears, so they can spread it before they know they have it. Measles starts with a high fever, cough, runny nose and red, watery eyes, followed a few days later by a rash that spreads from the face downward. It can lead to pneumonia, brain swelling and death, especially in young children, and outbreaks in the US are linked to communities where many people are not vaccinated.

Two doses of MMR vaccine give about 97% protection, and 1 dose gives about 93%. Check your family's records, especially before international travel; babies 6 to 11 months old who are traveling abroad should get an early dose before the trip, then the usual 2 doses later. If you are exposed and not immune, call your doctor quickly: an MMR dose within 72 hours or immune globulin within 6 days of exposure can prevent or soften the illness. If you or your child has a fever and rash, call ahead before going to a clinic or ER so staff can keep other patients, including babies and people with weak immune systems, from being exposed. Vitamin A and other supplements do not replace vaccination.

Ticks, mosquitoes and the illnesses they carry

Ticks and mosquitoes spread several infections in the US, and the risk depends on where you live, travel and spend time outdoors. Prevention is much the same for both: use an EPA-registered insect repellent containing DEET, picaridin, IR3535, oil of lemon eucalyptus, PMD or 2-undecanone, following the label. If you use sunscreen as well, apply it first. Do not use repellent on babies under 2 months, or oil of lemon eucalyptus on children under 3. Treat boots, clothing and gear with 0.5% permethrin, or buy pre-treated clothing, but never apply permethrin directly to skin.

Lyme disease

Lyme disease is a bacterial infection spread by blacklegged (deer) ticks, most often in the Northeast, mid-Atlantic and upper Midwest. A tick usually needs to be attached for 36 to 48 hours or more to pass on the infection, so daily tick checks make a real difference. After time outdoors, shower soon, check your whole body, your children and your pets, and tumble dry clothes on high heat for 10 minutes to kill ticks. Remove an attached tick with fine-tipped tweezers, grasping it close to the skin and pulling upward with steady pressure, then clean the area. Skip nail polish, petroleum jelly or heat.

An expanding red rash appears in about 70 to 80% of infections, 3 to 30 days after the bite. Many of these rashes are solid red rather than a bull's-eye. Fever, chills, headache, fatigue and joint aches are also common. Caught early, Lyme disease is usually treated with a short course of antibiotics; missed infections can spread to the joints, heart and nervous system.

West Nile virus

West Nile virus is the leading cause of mosquito-borne disease in the continental US, with most cases from summer through fall. About 8 in 10 infected people have no symptoms and about 1 in 5 develop fever and a flu-like illness, but about 1 in 150 develop a serious infection of the brain or spinal cord. People over 60 and those with certain medical conditions or weakened immune systems are at the highest risk. There is no vaccine for people and no specific treatment, so prevention is everything: use repellent, wear long sleeves and pants when mosquitoes are active, keep screens in good repair, and empty, scrub or cover anything that holds water, such as buckets, birdbaths, tires and plant saucers, once a week.

Tuberculosis

Tuberculosis (TB) is a bacterial infection that usually affects the lungs. It spreads through the air when someone with TB disease of the lungs or throat coughs, speaks or sings, usually after long periods of close contact rather than brief encounters. TB exists in two forms. In latent TB infection, the bacteria live in the body without causing symptoms, and the person cannot spread them, but the infection can turn into TB disease later. In TB disease, the bacteria are active, cause symptoms and can spread to others.

Testing is done with a TB blood test or skin test. Ask your doctor about testing if you were born in or lived in a country where TB is common, live or work in a shelter or correctional facility, or have been near someone with TB disease. HIV, diabetes and medicines that weaken the immune system raise the chance that latent infection becomes active. Treating latent infection, usually for 3 to 9 months depending on the regimen, prevents TB disease later. TB disease can be cured, but treatment takes months and every dose matters, because stopping early can lead to drug-resistant TB. Health departments often offer support such as directly observed therapy or video check-ins.

A cough that lasts 3 weeks or longer is the classic warning sign, especially with coughing up blood, chest pain, night sweats, fever or unexplained weight loss. Do not ignore a long-lasting cough just because it is not painful; see your doctor and mention any travel or contact with someone who had TB. If you are diagnosed with TB disease, your health department will help identify close contacts who should be tested and will advise when you can safely return to work or school.

People at higher risk and a plan for every season

Some people are much more likely to become seriously ill from common infections: babies and young children, adults 65 and older, pregnant people, people with chronic conditions such as heart disease, lung disease, diabetes or kidney disease, and people with weakened immune systems from cancer treatment, an organ transplant, HIV or medicines such as steroids. If you are in one of these groups, or care for someone who is, the steps in this guide matter more, and so does calling a doctor early in an illness, when treatments work best. In babies under 3 months, a fever of 100.4°F (38°C) or higher always needs a prompt call to the doctor.

When you talk to your doctor, mention recent travel, tick or mosquito bites, contact with someone who was sick, and your vaccine history, since these details often point to the diagnosis. Ask whether a test is needed, whether the illness is likely to be viral or bacterial, what treatment will and will not help, and when you can safely return to work or school. A simple seasonal routine keeps prevention from slipping.

• Late summer and fall: get your yearly flu vaccine, ideally by the end of October, ask about COVID-19 and RSV protection, and restock home tests, a thermometer and fever reducers.
• Winter: stay home when sick, improve indoor air, keep sick visitors away from newborns and older relatives, and call early for antivirals if you are at higher risk.
• Spring and summer: use repellent, check for ticks after time outdoors, empty standing water weekly and follow food safety rules for picnics and grilling.
• Before international travel: review MMR, hepatitis A and other travel vaccines at least a few weeks ahead.
• Every year: review vaccine records for the whole family at a checkup, and bring your list of medicines and conditions.

Every guide in this topic

• COVID-19: What the Evidence Actually Says
• What the Guidance Really Says About Flu Versus a Cold
• Measles: What the Evidence Actually Says
• Lyme Disease and Ticks: What Published Guidance Supports
• The Evidence on West Nile Virus and Mosquitoes, Without the Hype
• What the Guidance Really Says About Norovirus
• Strep Throat: What Published Guidance Supports
• Hepatitis A: Sorting the Evidence From the Noise
• The Evidence on Tuberculosis Basics, Without the Hype
• RSV (Respiratory Syncytial Virus): Sorting the Evidence From the Noise

Frequently asked questions

How long should I stay home when I have a respiratory illness?

The CDC advises staying home and away from others until your symptoms are improving overall and you have had no fever for 24 hours without fever-reducing medicine. For the next 5 days, take extra precautions such as wearing a mask, keeping your distance, washing your hands and improving indoor air, because you may still be able to spread the virus.

Why will my doctor not give me antibiotics for a cold or the flu?

Colds and flu are caused by viruses, and antibiotics only work against bacteria. Taking them for a viral illness will not help you recover faster, can cause side effects such as diarrhea and allergic reactions, and adds to antibiotic resistance. For flu, prescription antiviral medicines can help people at higher risk if started within about 48 hours of symptoms.

Is hand sanitizer as good as washing my hands?

Not always. Alcohol-based sanitizer works well against many germs when hands are not visibly dirty, but it does not work well against norovirus and some other germs. Washing with soap and water for at least 20 seconds is best after using the toilet, changing diapers, caring for someone sick and before preparing food. Use sanitizer when a sink is not available.

My home COVID-19 test was negative but I feel sick. What should I do?

Test again 48 hours later, because home antigen tests can miss early infections. Meanwhile, follow the general advice for respiratory illness: stay home until symptoms improve and you have been fever-free for 24 hours without medicine. If you are 65 or older or have a chronic condition, contact your doctor quickly, since antiviral treatment works best when started early.

How do I tell if a sore throat is strep?

You cannot tell for sure from symptoms alone. Strep is more likely with a sore throat that starts quickly, fever, painful swallowing and swollen tonsils or neck glands, without a cough or runny nose. A rapid strep test or throat culture confirms it. See your doctor or urgent care for a test, especially for a child aged 5 to 15.

I found a tick on my child. Do they need antibiotics?

Not usually. Remove the tick with fine-tipped tweezers, pulling upward with steady pressure, then clean the area. A tick usually needs to be attached for 36 to 48 hours or more to spread Lyme disease. Watch for a rash, fever, aches or fatigue over the following weeks, and call your doctor if they appear or if you live in an area where Lyme disease is common.

Am I protected against measles if I was vaccinated as a child?

If you received 2 doses of MMR vaccine, you are considered protected for life; 2 doses give about 97% protection. Adults born in 1957 or later who do not have records or other evidence of immunity need at least 1 dose. If you are unsure, ask your doctor, who can check your records or order a blood test.

How can I keep a stomach bug from spreading through my family?

Wash hands with soap and water often, since sanitizer works poorly against norovirus. Wear disposable gloves to clean up vomit or diarrhea, disinfect with a bleach solution or a product labeled for norovirus, and wash soiled laundry on a long hot cycle. Keep the sick person out of the kitchen while ill and for at least 2 days afterward.

Does a positive TB test mean I can spread tuberculosis?

Not necessarily. Many people with a positive test have latent TB infection, which causes no symptoms and cannot be spread to others. Your doctor will check for TB disease, usually with a chest X-ray and questions about symptoms. Treating latent infection, usually for 3 to 9 months, lowers the chance of developing TB disease later.

Which insect repellent should I use?

Choose an EPA-registered repellent containing DEET, picaridin, IR3535, oil of lemon eucalyptus, PMD or 2-undecanone, and follow the label. Apply sunscreen first if you use both. Do not use repellent on babies under 2 months, or oil of lemon eucalyptus on children under 3. Permethrin is for clothing and gear only, never for skin.

When to speak to a doctor

Call your doctor if an illness gets worse after starting to improve, a fever lasts more than a few days, a cold lasts more than 10 days, you have a sore throat with fever and no cold symptoms, a rash or fever follows a tick bite, a cough lasts 3 weeks or more, or vomiting and diarrhea last more than a few days or someone cannot keep fluids down. Call early if you are 65 or older, pregnant, or have a chronic condition or weakened immune system, and call promptly for any fever of 100.4°F (38°C) or higher in a baby under 3 months. Call ahead before visiting with a fever and rash. Call 911 for trouble breathing, chest pain or pressure, new confusion, a seizure, being unable to wake or stay awake, bluish or gray lips or skin, a severe headache with a stiff neck, signs of severe dehydration, or, in a child, fast breathing or ribs pulling in with each breath.

Where this comes from

• CDC — Respiratory Virus Guidance
• CDC — Treatment of Respiratory Illnesses
• CDC — About Measles
• CDC — About RSV
• CDC — About Norovirus
• CDC — Hepatitis A Basics
• CDC — About Strep Throat
• CDC — Lyme Disease
• CDC — About West Nile
• CDC — About Tuberculosis

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.