Mayo Clinic Health Library

Infectious diseases

Updated: 01-23-2013

Definition

Infectious diseases are disorders caused by organisms — such as bacteria, viruses, fungi or parasites. Many organisms live in and on our bodies. They're normally harmless or even helpful, but some organisms under certain conditions may cause disease.

Some infectious diseases can be passed from person to person. Some, however, are transmitted via bites from insects or animals. Others are acquired by ingesting contaminated food or water or other exposures in the environment.

Signs and symptoms vary, but often include fever and chills. Mild complaints may respond to home remedies, while some life-threatening infections may require hospitalization.

Many infectious diseases, such as measles and chickenpox, can be prevented by vaccines. Frequent and thorough hand-washing also helps protect you from infectious diseases.

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Symptoms

Each infectious disease has its own specific signs and symptoms. General signs and symptoms common to many infectious diseases include:

  • Fever
  • Loss of appetite
  • Fatigue
  • Muscle aches

When to see a doctor
You should seek medical attention if you:

  • Have been bitten by an animal
  • Are having trouble breathing
  • Have been coughing for more than a week
  • Have severe headache with fever or seizures with fever
  • Experience a rash or swelling
  • Have unexplained fever
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Causes

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. Even smaller than bacteria, viruses cause a multitude of diseases — ranging from the common cold to AIDS.
  • Fungi. Many skin diseases, such as ringworm and athlete's foot, are caused by fungi. Other types of fungi can infect your lungs or nervous system.
  • Parasites. Malaria is caused by a tiny parasite that is transmitted by a mosquito bite. Other parasites may be transmitted to humans from animal feces.

Direct contact
An easy way to catch most infectious diseases is by coming in contact with a person or animal who has the infection.Three ways infectious diseases can be spread through direct contact are:

  • Person to person. The most common way for infectious diseases to spread is through the direct transfer of bacteria, viruses or other germs from one person to another. This can occur when an individual with the bacterium or virus touches, coughs on or kisses someone who isn't infected. These germs can also spread through the exchange of body fluids from sexual contact or a blood transfusion. The person who passes the germ may have no symptoms of the disease, but may simply be a carrier.
  • Animal to person. Pets can carry many germs. Being bitten or scratched by an infected animal can make you sick and, in extreme circumstances, can be fatal. Handling animal waste can be hazardous, too. For example, you can acquire a toxoplasmosis infection by scooping your cat's litter box.
  • Mother to unborn child. A pregnant woman may pass germs that cause infectious diseases to her unborn baby. Some germs can pass through the placenta. Germs in the vagina can be transmitted to the baby during birth.

Indirect contact
Disease-causing organisms also can be passed by indirect contact. Many germs can linger on an inanimate object, such as a tabletop, doorknob or faucet handle. When you touch a doorknob handled by someone ill with the flu or a cold, for example, you can pick up the germs he or she left behind. If you then touch your eyes, mouth or nose before washing your hands, you may become infected.

Insect bites
Some germs rely on insect carriers — such as mosquitoes, fleas, lice or ticks — to move from host to host. These carriers are known as vectors. Mosquitoes can carry the malaria parasite or West Nile virus, and deer ticks may carry the bacterium that causes Lyme disease.

Food contamination
Another way disease-causing germs can infect you is through contaminated food and water. This mechanism of transmission allows germs to be spread to many people through a single source. E. coli, for example, is a bacterium present in or on certain foods — such as undercooked hamburger or unwashed fruits or vegetables.

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Risk factors

While anyone can catch infectious diseases, you may be more likely to get sick if your immune system isn't working properly. This may occur if:

  • You're taking steroids or other medications that suppress your immune system, such as anti-rejection drugs for a transplanted organ
  • You have HIV or AIDS
  • You have certain types of cancer or other disorders that affect your immune system

In addition, certain other medical conditions may predispose you to infection, including implanted medical devices, malnutrition and extremes of age, among others.

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Complications

Most infectious diseases have only minor complications, but some infections — such as pneumonia, AIDS or meningitis — can become life-threatening. A few types of infections have been linked to a long-term increased risk of cancer:

  • Human papillomavirus is linked to cervical cancer
  • Hepatitis B and C increase the risk of liver cancer
  • Helicobacter pylori is linked to stomach cancer
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Preparing for your appointment

If you call your family doctor, he or she may refer you to a specialist, depending on which of your organ systems is affected by the infection. For example, a dermatologist specializes in skin conditions, while 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 medications and dietary supplements you take
  • Questions you want to ask the doctor

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

  • What is likely causing my symptoms or condition?
  • Other than the most likely cause, what are other possible causes for my symptoms or condition?
  • What kinds of tests do I need?
  • Is my condition likely temporary or chronic?
  • What is the best course of action?
  • What are the alternatives to the primary approach you're suggesting?
  • I have these other health conditions. How can I best manage them together?
  • Are there any restrictions that I need to follow?
  • Should I see a specialist?
  • Is there a generic alternative to 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?

Don't hesitate to ask any other questions that occur to you.

What to expect from your doctor
Your doctor is likely to ask you a number of questions, including:

  • When did you begin experiencing symptoms?
  • Have your symptoms been continuous or occasional?
  • How severe are your symptoms?
  • Have you recently come into contact with anyone who's sick?
  • Have you been bitten or scratched by an animal or 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?
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Tests and diagnosis

Your doctor may order lab work or imaging scans to help determine what's causing your symptoms.

Laboratory tests
Many infectious diseases have similar signs and symptoms. Samples of your body fluids can sometimes reveal evidence of the particular microbe that's causing your illness. This helps your doctor tailor your treatment.

  • Blood tests. A technician obtains a sample of your blood by inserting a needle into a vein in your arm or hand. This test may be slightly uncomfortable for some people but usually takes only a few minutes.
  • Urine tests. This painless test requires you to urinate into a container. To avoid potential contamination of the sample, you may be instructed to cleanse your genital area with an antiseptic pad and to collect the urine midstream.
  • Throat swabs. Samples from your throat, or other moist areas of your body, often are obtained with a sterile swab.
  • Spinal tap (lumbar puncture). This procedure obtains a sample of your cerebrospinal fluid through a needle carefully inserted between the bones of your lower spine. In most cases, you'll be asked to lie on your side with your knees pulled up toward your chest. This test can be uncomfortable and you might develop a headache afterward.

Imaging scans
Imaging procedures — such as X-rays, computerized tomography and magnetic resonance imaging — can help pinpoint diagnoses and rule out other conditions that may be causing your symptoms.

  • X-rays. This painless procedure exposes a part of your body to a small dose of radiation to produce an image of the structures inside your body. A chest X-ray, for example, can reveal signs of pneumonia.
  • Computerized tomography (CT). CT scans digitally combine X-rays taken from many different angles to produce cross-sectional images of bones, organs and other soft tissues. CT images reveal more details than do regular X-rays.
  • Magnetic resonance imaging (MRI). MRI uses radio waves and a strong magnetic field to produce detailed images of internal structures. This test involves lying on a narrow table that slides into a tunnel within the MRI machine. Some people find the enclosed space claustrophobic, but medications can help you relax and can make the MRI experience easier.

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

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Treatments and drugs

Knowing what type of germ is causing your illness makes it easier for your doctor to choose appropriate treatment.

Antibiotics
Antibiotics are grouped into "families" of similar types. Bacteria also are put together in groups of similar types, such as streptococcus or E. coli. Certain types of bacteria are especially susceptible to particular classes of antibiotics. So treatment can be targeted more precisely if your doctor knows what type of bacteria you're fighting.

Antibiotics are reserved for bacterial infections, because these types of drugs have no effect on illnesses caused by viruses. But sometimes it's difficult to tell which type of germ is at work. For example, some types of pneumonia are caused by viruses while others are caused by bacteria.

The overuse of antibiotics has resulted in several types of bacteria developing resistance to one or more varieties of antibiotics. This makes these bacteria much more difficult to treat.

Antivirals
Drugs have been developed to treat some, but not all, viruses. Examples include the viruses that cause:

  • AIDS
  • Herpes
  • Hepatitis B
  • Hepatitis C
  • Influenza

Antifungals
Severe fungal infections can affect the lungs or the mucous membranes of the mouth and throat — most commonly in people who have weakened immune systems. Antifungals are the drugs of choice for these types of infections.

Anti-parasitics
Some diseases, including malaria, are caused by tiny parasites. While there are drugs to treat these diseases, some varieties of parasites have developed resistance to the drugs.

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Lifestyle and home remedies

Many infectious diseases, such as colds, will resolve on their own. Drink plenty of fluids and get lots of rest.

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Prevention

Infectious agents can enter your body through:

  • Skin contact or injuries
  • Inhalation of airborne germs
  • Ingestion of contaminated food or water
  • Tick or mosquito bites
  • Sexual contact

Follow these tips to decrease your risk of infecting yourself or others:

  • Wash your hands. This is especially important before and after preparing food, before eating and after using the toilet.
  • Get vaccinated. Immunization can drastically reduce your chances of contracting many diseases. Make sure to keep your recommended vaccinations, as well as your children's, up to date.
  • Stay home. Don't go to work if you're vomiting, have diarrhea or are running a fever. Don't send your child to school if he or she has these signs and symptoms, either.
  • Prepare food safely. Keep counters and other kitchen surfaces clean when preparing meals. In addition, promptly refrigerate leftovers — don't let cooked foods remain at room temperature for extended periods of time.
  • Practice safe sex. Use condoms if you or your partner has a history of sexually transmitted infections or high-risk behavior.
  • Don't share personal items. Use your own toothbrush, comb and razor. Avoid sharing drinking glasses or dining utensils.
  • Travel wisely. Don't fly when you're ill. With so many people confined to a small area, you may infect other passengers on the plane. And your trip won't be comfortable, either. If you're traveling out of the country, talk to your doctor about any special vaccinations you may need.
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