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

• Fever
• Diarrhea
• Fatigue
• Muscle aches
• Coughing

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
• Experience a rash or swelling
• Have unexplained or prolonged 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. 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 an animal with the infection. Infectious diseases can be spread through direct contact such as:

• Person to person. Infectious diseases commonly spread through the direct transfer of bacteria, viruses or other germs from one person to another. This can happen when an individual with the bacterium or virus touches, kisses, or coughs or sneezes on someone who isn't infected.

  These germs can also spread through the exchange of body fluids from sexual contact. The person who passes the germ may have no symptoms of the disease, but may simply be a carrier.

• Animal to person. Being bitten or scratched by an infected animal — even a pet — can make you sick and, in extreme circumstances, can be fatal. Handling animal waste can be hazardous, too. For example, you can get 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 or through breast milk. Germs in the vagina can also 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. Deer ticks may carry the bacterium that causes Lyme disease.

Food contamination

Disease-causing germs can also infect you through contaminated food and water. This mechanism of transmission allows germs to be spread to many people through a single source. Escherichia coli (E. coli), for example, is a bacterium present in or on certain foods — such as undercooked hamburger or unpasteurized fruit juice.

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.

Most infectious diseases have only minor complications. But some infections — such as pneumonia, AIDS and 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
• Helicobacter pylori is linked to stomach cancer and peptic ulcers
• Hepatitis B and C have been linked to liver cancer

In addition, some infectious diseases may become silent, only to appear again in the future — sometimes even decades later. For example, someone who's had chickenpox may develop shingles much later in life.

Follow these tips to decrease the risk of infection:

• Wash your hands. This is especially important before and after preparing food, before eating, and after using the toilet. And try not to touch your eyes, nose or mouth with your hands, as that's a common way germs enter the body.
• Get vaccinated. Vaccination can drastically reduce your chances of contracting 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 child to school if he or she has these signs, either.

• Prepare food safely. Keep counters and other kitchen surfaces clean when preparing 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 promptly refrigerate leftovers — don't let cooked foods remain at room temperature for long periods of time.

• Practice safe sex. Always 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. If you're traveling out of the country, talk to your doctor about any special vaccinations — such as yellow fever, cholera, hepatitis A or B, or typhoid fever — you may need.

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 body fluids can sometimes reveal evidence of the particular microbe that's causing the illness. This helps the doctor tailor treatment.

• Blood tests. A technician obtains a sample of blood by inserting a needle into a vein, usually in the arm.
• 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 the throat, or other moist areas of the body, may be obtained with a sterile swab.
• Stool sample. You may be instructed to collect a stool sample so a lab can check the sample for parasites and other organisms.
• Spinal tap (lumbar puncture). This procedure obtains a sample of the cerebrospinal fluid through a needle carefully inserted between the bones of the lower spine. You'll usually be asked to lie on your side with your knees pulled up toward your chest.

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 symptoms.

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.

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. Treatment can be targeted more precisely if your doctor knows what type of bacteria you're infected with.

Antibiotics are usually 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, pneumonia can be caused by a bacterium, a virus, a fungus or a parasite.

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:

• HIV/AIDS
• Herpes
• Hepatitis B
• Hepatitis C
• Influenza

Antifungals

Topical antifungal medications can be used to treat skin or nail infections caused by fungi. Some fungal infections, such as those affecting the lungs or the mucous membranes, can be treated with an oral antifungal. More-severe internal organ fungal infections, especially in people with weakened immune systems, may require intravenous antifungal medications.

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.

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

A number of products have claimed to help fend off common illnesses, such as the cold or flu. While some of these substances have appeared promising in early trials, follow-up studies may have had conflicting or inconclusive results. More research needs to be done.

Some of the substances that have been studied for preventing or shortening the duration of infection include:

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

Check with your doctor before trying any products that promise to boost your immune system or chase colds and other illnesses away. Some of these products may cause allergic reactions or interact adversely with other medications you may be taking.

You'll probably first see your primary care doctor. Depending on the severity of your infection, as well as which of your organ systems is affected by the infection, your doctor may refer you to a specialist. For example, a dermatologist specializes in skin conditions, and 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's the most likely cause of my symptoms?
• Are there other possible causes for my symptoms?
• 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 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?

What to expect from your doctor

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

• When did your symptoms begin?
• Do your symptoms come and go, or do you have symptoms all the time?
• 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?