The Nature of Germs and Infection

A germ is a microscopic organism that can cause disease. The term "microscopic organism" means that it is so small it can be seen only with a microscope.

Body Defenses

The human body constantly faces attack from foreign invaders that can cause infection and disease. These invaders range from living microbes (MY-krobes), such as bacteria*, fungi*, parasites*, and viruses*, to nonliving toxins, chemicals, and drugs.

Signs and Symptoms

Although the words may sound interchangeable, doctors use the word "symptom" to mean something different from the word "sign." A symptom is anything a patient experiences or feels, such as a headache, dizziness, or the sensation of nausea (NAW-zee-uh). A sign, on the other hand, is something that can be noted by a doctor during a physical examination, such as elevated blood pressure, fever, or swollen elbow.

Laboratory Tests

Before doctors can treat someone, they have to understand what is causing the illness. By talking with the patient ("taking the history"), examining the patient, and, when necessary, ordering tests (often called laboratory or lab tests), a doctor is better able to determine the illness.


Throughout life, every person will have at least one infection and more than likely, dozens of them. Fortunately, in many cases these infections will be relatively minor and will not require special treatment.

Public Health

People often think of health and illness in terms of a single person contracting an infection such as the flu by being exposed to the virus*, or a person trying to avoid an infection by careful hand washing or getting a shot. However, there are other, broader ways to look at health and illness that go beyond just one individual person.

Vaccination (Immunization)

Before about 1800, a dangerous disease known as smallpox killed millions of people throughout the world. All that was soon to change as the result of an observation made by Edward Jenner, an English country doctor, in 1796.


An abscess develops when the body's immune system isolates an area of body tissue that has been infected by an invading microorganism* (usually bacteria) to prevent the infection from spreading further into the body. It does this by sending infection-fighting leukocytes (LOO-kuh-sites) to the infected area; leukocytes are specialized white blood cells that can destroy infectious microorganisms such as bacteria, parasites, and viruses.

AIDS and HIV Infection

AIDS is the disease caused by human immunodeficiency virus type 1, or HIV-1 (usually referred to as HIV). HIV belongs to the retrovirus family, a group of viruses that have the ability to use cells' machinery to replicate (make more copies of the infecting virus).


Anthrax (AN-thraks) is a rare infectious disease caused by the bacterium Bacillus anthracis.

Arthritis, Infectious

Most of the time, bacteria cause infectious arthritis. This form of arthritis is also called septic arthritis, and the infected joint is referred to as a "septic joint." Staphylococcus (stah-fih-lo-KAH-kus) or Streptococcus (strep-tuh-KAH-kus) bacteria are the culprits in most cases of septic arthritis.


Also known as biological warfare, bioterrorism is a form of warfare that uses specific microorganisms*, such as harmful bacteria and viruses, to cause illness or death deliberately in people or animals. When organisms are used in this way, they become weapons.


There are seven types of botulinum toxin, each designated by a letter from A through G. Only four types (A, B, E, and F) make people sick.


Bronchiolitis is caused by a virus that infects the bronchioles, the smallest airways that carry air through the lungs. The linings of these airways swell and become blocked with fluid and mucus*, making it difficult to breathe.

Bronchitis, Infectious

When a person has bronchitis, the tissue lining the airways swells, narrowing the air passages and making it difficult to breathe. The inflamed airways produce larger amounts of a thick, slippery substance called mucus (MYOO-kus), which can clog the air passages and complicate breathing even more.

Cat Scratch Disease

Cat scratch disease (also called cat scratch fever) is caused by the bacterium Bartonella henselae (bar-tuh-NEH-luh HEN-suh-lay), which is found in the saliva of cats and kittens all over the world. About 3 to 10 days after a person is bitten or scratched by a cat, a blister or small bump may develop.

Chiamydial Infections

There are different types of chlamydia bacteria. Chlamydia trachomatis (kla-MIH-dee-uh truh-KO-mah-tis) is the bacterium that causes genital (and sometimes throat) infections.


Cholera is an illness caused by the bacterium Vibrio cholerae, which is contracted by eating contaminated food or drinking contaminated water. The bacteria can cause serious diarrhea by producing a toxin that makes the intestines release more water and minerals than usual.

Chronic Fatigue Syndrome

Chronic fatigue syndrome is a condition that causes almost constant exhaustion. People with CFS cannot just get more sleep to feel better, because their fatigue does not improve with rest.

Coccidioidomycosis (Valley Fever)

During World War II, American trainees sent to Arizona and parts of southern California for flight training took thousands of days of sick leave because of coccidioidomycosis, a disease caused by Coccidioides immitis (kok-sih-dee-OYD-eez IH-mih-tus), a fungus that hibernates a few inches beneath semi-dry soil. The disease's other name, valley fever, comes from the San Joaquin Valley region of California, where the fungus was first identified.