Infectious diseases are illnesses that happen when tiny living beings get inside the body and multiply. Bacteria, viruses, fungi and parasites are examples of organisms that can cause illness. Many organisms live in and on our bodies. They're too small to see with the eyes. And they're often harmless or even helpful. But under certain conditions, some organisms can cause disease.

Some infectious diseases can pass from person to person. Some spread through insects or other animals. You may get others by eating contaminated food or water. Or you could be exposed to germs around you, such as those on surfaces.

Symptoms of infectious diseases vary depending on the cause of the infection. But they often include fever and tiredness. Rest and home remedies may clear up mild infections. But other infections can be life-threatening, and treatment could involve hospital care.

Many infectious diseases, such as measles and chickenpox, can be prevented by vaccines. And other illnesses, such as COVID-19, might make you less sick if you get vaccinated. Frequent and thorough hand-washing also helps protect you from most infectious diseases.

Each infectious disease has its own specific symptoms. But some illnesses share general symptoms, such as:

• Fever.
• Faster heartbeat.
• Diarrhea.
• Tiredness, also called fatigue.
• Muscle aches.
• Coughing.
• Night sweats.

Call your healthcare professional if you:

• Have been bitten by an animal.
• Have trouble breathing.
• Cough for more than a week, or you cough up yellow-green mucus or blood.
• Have severe headache with fever.
• Get a rash that's widespread, comes with a fever, leaks fluid, or feels painful or warm.
• Have unexplained or long-lasting fever.
• Have sudden vision problems.

Infectious diseases can be caused by:

• Bacteria. These one-cell organisms are responsible for illnesses such as strep throat, urinary tract infections and tuberculosis.
• Viruses. These microscopic germs are even smaller than bacteria. They cause many diseases, including the common cold, COVID-19 and HIV.
• Fungi. These cause many skin diseases, such as ringworm and athlete's foot. Other types of fungi can infect the lungs, eyes, liver or brain.
• Parasites. The disease malaria is caused by a tiny parasite that is spread by a mosquito bite. Other parasites may be spread to people from animal feces.

Direct contact

An easy way to catch most infectious diseases is by coming in contact with an infected person or animal. These diseases can be spread through direct contact such as:

• Person to person. This can happen when an infected person touches, kisses, or coughs or sneezes on someone who isn't infected.

  These germs also can spread through body fluids during sexual contact. The person who passes the germ may have no symptoms.

• Animal to person. Being bitten or scratched by an infected animal — even a pet — can make you sick. In extreme situations, it can be deadly. Handling animal waste can raise the risk of catching a disease too. For example, you could get an infection from a parasite by scooping your cat's litter box without wearing disposable gloves or washing afterward. Pregnant people and others with weakened immune systems are especially at risk of this infection, called toxoplasmosis.
• Mother to unborn child. A pregnant person may pass germs that cause infectious diseases to the unborn baby. Some germs can pass through the organ that sends oxygen and nutrients to an unborn child, called the placenta. Germs in the vagina also can be spread to the baby during birth. There also is a small risk of germs passing through breast milk. This risk is higher if you buy breast milk online from a milk-sharing site, or if your baby is fed someone else's breast milk.

Indirect contact

Disease-causing germs also can be passed by indirect contact. Many germs can linger on surfaces such as tabletops, doorknobs or faucet handles.

For example, say you touch a doorknob that's been handled by someone ill with the flu or a cold. You could pick up the germs that person left behind. If you then touch your eyes, mouth or nose before washing your hands, you may become infected.

Bug bites

Bugs such as mosquitoes, fleas, lice or ticks can harbor germs and spread them to people with their bites. Mosquitoes can carry the malaria parasite or West Nile virus. Deer ticks may carry the bacterium that causes Lyme disease.

Food contamination

Disease-causing germs also can infect you through contaminated food and water. For example, the bacterium Escherichia coli (E. coli) is present in or on foods such as undercooked hamburger or unpasteurized fruit juice.

Anyone can catch infectious diseases. But you may be more likely to get sick if your immune system isn't working properly. This may happen if:

• You take steroids or other medicines that suppress your immune system, such as chemotherapy for cancer.
• You have HIV or AIDS.
• You have certain types of cancer or other conditions that affect your immune system.

Other risk factors include malnutrition and hospital procedures that insert tubes in a vein to give medicine or other treatments.

Some infectious diseases cause minor health problems, such as the common cold. Others can become life-threatening, such as pneumonia and meningitis. A few types of infections have been linked to a long-term higher risk of cancer:

• Human papillomavirus is linked to cervical cancer.
• Helicobacter pylori is tied to stomach cancer and peptic ulcers.
• Hepatitis B and C have been linked to liver cancer.

The germs that cause some infectious diseases also may become inactive, only to cause problems again in the future. For example, someone who's had chickenpox may develop shingles much later in life.

Follow these tips to lower the risk of infection:

• Wash your hands. Scrub them with water and soap for at least 20 seconds. Or use a hand sanitizer with at least 60% alcohol. Washing is very important before and after you prepare food, before you eat, and after you use the toilet or touch garbage. It's also key to wash after you handle pets, their food or their waste. Be sure to scrub your hands after you care for someone who's sick, before and after you treat a cut, and after you blow your nose too. Also, try not to touch your eyes, nose or mouth with your hands. That's a common way for germs to enter the body.
• Get vaccinated. Vaccination can greatly lower your chances of getting many diseases. Make sure to keep up to date on your recommended vaccinations, as well as your children's.
• Stay home when ill. Don't go to work if you are vomiting, have diarrhea or have a fever. Don't send your children to school if they have these symptoms, either. And wear a mask if you're sick or if you need to take care of someone who's sick with COVID-19.
• Prepare food safely. Keep counters and other kitchen surfaces clean when you prepare meals. Cook foods to the proper temperature, using a food thermometer to check for doneness. For ground meats, that means at least 160 F (71 C); for poultry, 165 F (74 C); and for most other meats, at least 145 F (63 C). Also, quickly refrigerate leftovers — don't let cooked foods remain at room temperature for long periods of time.
• Have safe sex. Use a condom each time you have sex if you or your partner has a history of sexually transmitted diseases (STDs). It's also important to use condoms if either of you has more than one sex partner. Be sure to get tested for STDs, and share the results with each other.
• Don't share personal items. Use your own toothbrush, comb and razor. Don't share drinking glasses or dining utensils, especially if someone seems sick.
• Travel wisely. If you're traveling out of the country, talk to your healthcare professional about any special vaccinations you may need. Examples include vaccines for yellow fever, cholera, hepatitis A or B, or typhoid fever.

Your healthcare professional will likely ask you about your symptoms and medical history. You'll likely get a physical exam too. You also may need certain tests or imaging scans to help figure out the cause of your symptoms.

Lab tests

Many infectious diseases have symptoms in common. Samples of body fluids can sometimes point to the particular germ that causes the illness. This helps your care team make sure you get the right treatment.

• Blood tests. A healthcare professional inserts a needle into a vein, often in the arm.
• Urine tests. This involves urinating into a container.
• Throat swabs. Samples from the throat, or other moist areas of the body, may be taken with a sterile swab.
• Stool sample. You may be told to collect a stool sample in a container, so a lab can check for parasites and other organisms.
• Spinal tap. Also called lumbar puncture, this procedure uses a needle to take a sample of the fluid that surrounds the brain and spinal cord. A needle is carefully placed between the bones of the lower spine. You'll likely be asked to lie on your side with your knees pulled up toward your chest.

Imaging scans

Tests that make images of the inside of the body also can help your healthcare team figure out the cause of your symptoms. Imaging tests include X-ray, computerized tomography, ultrasound and MRI.

Biopsies

During a biopsy, a tiny sample of tissue is taken for testing. For example, a biopsy of lung tissue can be checked for a variety of fungi that can cause a type of pneumonia.

Knowing what type of germ is causing your illness makes it easier for your healthcare team to choose the right treatment.

Antibiotics

Antibiotics are medicines that clear up illnesses caused by bacteria. They are grouped into classes of similar types. Bacteria also are put together in groups of similar types, such as streptococcus or E. coli. Some types of bacteria are especially prone to certain classes of antibiotics. Treatment can be targeted more precisely if your healthcare team knows what type of bacteria you're infected with.

Antibiotics have no effect on illnesses caused by viruses. But sometimes it's hard to tell which type of germ is at work. For example, pneumonia can be caused by a bacterium, a virus, a fungus or a parasite.

The overuse of antibiotics is a problem. Bacteria can change in response to these medicines. As a result, some types of bacteria can resist the effects of one or more varieties of antibiotics. This is called antibiotic resistance. It makes these germs much harder to treat.

Antivirals

These medicines can treat certain viruses. Examples include the viruses that cause:

• COVID-19.
• HIV.
• Herpes.
• Hepatitis B.
• Hepatitis C.
• Influenza.

Antifungals

Topical antifungal medications that you put on the body can treat skin or nail infections caused by fungi. Some fungal infections, such as those that affect the lungs or the mucous membranes, can be treated with an oral antifungal that you take by mouth. Worse infections of the internal organs may need to be treated with intravenous antifungal medicines. You receive these through a needle in a vein at the hospital.

Anti-parasitics

Some diseases, including malaria, are caused by tiny parasites. These organisms live in or on host organisms, which can include people. Parasites get their nutrients from or at the expense of their hosts. Medicines can treat diseases caused by parasites. But some types of parasites have developed resistance to the drugs.

Many infectious diseases, such as colds, go away on their own. Drink plenty of fluids and get lots of rest. If other people live in your home, try not to have close contact with them while you're sick. Think about wearing a mask around them.

Many products have claimed to help fend off common illnesses, such as the cold or flu. Some of these substances have appeared promising in early studies. But follow-up research may have had unclear results. In general, more research needs to be done.

Some of the substances that have been studied for preventing or shortening the length of infections include:

• Cranberry.
• Echinacea.
• Garlic.
• Ginseng.
• Vitamin C.
• Vitamin D.
• Zinc.

Check with your healthcare team before you try any products that promise to boost your immune system or chase away colds and other illnesses. Some of these products may cause side effects or mix poorly with other medicines you may be taking.

You'll probably first see your primary care team. You may get referred to a specialist, depending on how bad your infection is and what parts of the body it affects. For example, a dermatologist treats skin conditions. A pulmonologist treats lung disorders.

What you can do

You may want to write a list that includes:

• Detailed descriptions of your symptoms.
• Information about medical problems you've had.
• Information about your parents' or siblings' medical problems.
• All the medicines and dietary supplements you take.
• Questions you want to ask your healthcare professional.

Preparing a list of questions for your healthcare professional will help you make the most of your time together. For infectious diseases, some basic questions to ask include:

• What's the most likely cause of my symptoms? Are there other possible causes?
• What kinds of tests do I need?
• Is my condition likely temporary or long lasting?
• What treatment do you recommend?
• I have other health conditions. How can I best manage these conditions together?
• Is there a generic version of the medicine you're prescribing?
• Are there any brochures or other printed material that I can take home with me? What websites do you recommend?

What to expect from your doctor

Your healthcare professional is likely to ask you a number of questions, including:

• When did your symptoms begin, and how serious are they?
• Do your symptoms come and go, or do you have symptoms all the time?
• Have you recently come into contact with anyone who's sick?
• Have you been bitten or scratched by an animal? Or have you come into contact with animal feces?
• Do you have any insect bites?
• Have you eaten undercooked meat or unwashed vegetables?
• Have you been out of the country recently?