Infectious diseases are all caused by infective agents — such as bacteria, viruses, fungi or parasites. 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 of infectious diseases vary, but often include fever and chills. Mild complaints may respond to home remedies, while some life-threatening infections may require hospitalization and intravenous antibiotics.

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

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

Infectious diseases can be caused by:

• Bacteria. These one-cell organisms are responsible for such illnesses as strep throat, urinary tract infections and tuberculosis.
• Viruses. Even smaller than bacteria, viruses are the cause of a multitude of diseases — ranging from the common cold to AIDS.
• Fungi. Many skin diseases, such as ringworm or 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 someone who has one. This "someone" can be a person or an animal or for an unborn baby, its mother. Three different ways infectious disease 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.
• Animal to person. Your household pet might seem harmless, but pets can carry many germs. Being bitten or scratched by an infected animal can make you sick and, in extreme circumstances, could even cause death. 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 the birthing process.

Indirect contact
Disease-causing organisms can also be passed along by indirect contact. Many germs can linger on an inanimate object, such as a tabletop, doorknob or faucet handle. When you touch the same doorknob grasped 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 is a bacterium present in or on certain foods — such as undercooked hamburger or unwashed fruits or vegetables.

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

• You have an autoimmune disease, such as rheumatoid arthritis or lupus
• You're receiving treatment for cancer
• You're taking steroids, which affect your immune system
• You take anti-rejection drugs for a transplanted organ
• You have HIV or AIDS

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 an increased risk of cancer. Examples include:

• Human papillomavirus is linked to cervical cancer
• Hepatitis B and C increases the risk of liver cancer
• Helicobacter pylori is linked to stomach cancer

While you may initially consult your family physician, 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 the medical problems of your parents or siblings
• All the medications and dietary supplements you take
• Questions you want to ask the doctor

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 more precisely.

• Blood tests. A technician obtains a sample of your blood with a needle inserted 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 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 (CT) or 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.

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 cause a specific 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 in "families" of similar types. Bacteria are also grouped into distinct families, such as streptococcus or E. coli. Certain families 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.

Many infectious diseases can be treated successfully at home. Make sure to drink plenty of fluids and to get lots of rest.

Infectious agents can enter your body through:

• Skin contact
• 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 are 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 diseases or high-risk behavior — or abstain altogether.
• Don't share personal items. Use your own toothbrush, comb and razor blade. 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. Depending on where your travels take you, talk to your doctor about any special immunizations you may need.