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For as long as humans have been on this planet, the microscopic life forms that we call germs - bacteria, viruses, protozoa, and others - have been here, too. For all this time, they have been growing and multiplying in the soil, in the water, and in our bodies. Most of the time, people and germs coexist peacefully; in fact, humans couldn't live in a germ-free world. But germs are also the source of many human illnesses.

Kinds of germs

Let's get to know germs better. What do they look like and how do they live? How do they harm us and how do medications work against them?


Bacteria are living things made of only one cell. Bacteria weigh about 1 trillionth of an ounce each, and they come in three basic shapes: cocci, shaped like small balls or spheres; bacilli, shaped like rods or sticks of chewing gum; and small spirals called spirochetes.

Under a microscope, scientists can often identify different species of bacteria by their shape, their grouping pattern, and the purple or pink color that they pick up from a special laboratory procedure called a Gram's stain. For example, staphylococci bacteria from pimples would look like clusters of purple grapes, while Escherichia coli bacteria from the intestines would appear as single pink rods.

Besides having a characteristic appearance under the microscope, each species of bacteria also has a typical bacterial lifestyle. For example, some bacteria colonize a particular part of a person's body and remain there, often without causing illness. Some may even help the body function, as in the case of intestinal bacteria. Other bacteria prefer the soil and live there harmlessly (like the bacteria that causes tetanus) - until they find themselves trapped inside a dirty cut on a gardener's hand. Bacteria differ in their need for oxygen, too, with some living only where there is air (aerobic bacteria) and others living without oxygen (anaerobic bacteria).

Of the approximately 1,600 species of known bacteria, less than 200 are pathogenic (disease-causing). Pathogenic bacteria can cause illness in at least three different ways: by invasive action, directly invading and attacking a part of the body; by making toxins, chemical byproducts that act as poisons; or by multiplying into large clumps that block tiny blood vessels or interfere with the normal closing of heart valves. Depending on the type of bacteria and the illness it causes, bacteria can spread in the following ways:

  •  through contaminated water and food
  •  in the tiny fluid droplets of coughs and sneezes
  •  through dirty hands
  •  through contaminated surfaces
  •  in a sick person's body fluids


On a microscopic scale, viruses are midgets compared with bacteria. Since most viruses are only 1/100 to 1/1000 the size of most bacteria, there are actually some forms of viruses that prey on bacteria instead of animals or humans. All viruses are submicroscopic, meaning that you can't see them with a standard laboratory microscope. In fact, it was not until the electron microscope was invented in the 1940s that scientists had any notion of what even the largest virus looked like.

On the outside, viruses come in several different shapes. Some are 20-sided polygons, others are rod-shaped, and some even appear to have a head and a tail. On the inside, viruses contain a core of nucleic acids, either DNA or RNA, surrounded by one or two protein shells called capsids. Many viruses also have an outer envelope made of a mixture of proteins, fats, and carbohydrates. Scientists classify viruses according to the type of nucleic acid they contain (DNA viruses or RNA viruses), their capsid composition, the presence or absence of an envelope, and the way they reproduce.

Viruses can only "live," grow, and reproduce inside other living cells. Outside of living cells, viruses are just structured and complex - but inactive - collections of molecules. Whenever a virus rests on the surface of a dirty glass or floats in the droplets of a contaminated sneeze, it is technically "lifeless." Yet once a person swallows it or breathes it in, that same virus can invade the person's cells, reproduce, and make the person sick. Viruses do this by replication. They invade living cells and use the cells' internal chemical reactions to duplicate their viral nucleic acids and proteins. Then they assemble new viruses and release them to infect other helpless cells nearby.

Many viruses invade the body by crossing the moist mucous membranes that line the breathing passages, eyes, intestines, or reproductive tracts. They get there by traveling in an infected person's body fluids or in the airborne droplets of coughs and sneezes. Once inside the body, viruses can spread from one area to another through the blood vessels and arteries, the fluid-filled channels between tissues, and even along the length of nerve cells. Once they invade and cause illness, some forms of viruses can be totally defeated by the immune system. Others, like the chicken pox virus and herpes simplex virus, can lie dormant in our cells and cause more than one episode of illness, even years after infection.


The fungi are a group of about 50,000 species of simple plants that cannot manufacture their own food from the soil, water, and air around them. Instead, they must draw nutrition from nearby plants and animals, living or dead. Some fungi, such as mushrooms, are very large and complex, but others, such as yeasts, exist as single cells.

Because fungi prefer warm, damp areas, they love to grow on moist parts of our bodies; athlete's foot, and vaginal "yeast" infections are two common fungal infections. Since our breathing passages are moist, some forms of fungi can invade them as well.

Persons whose immune systems are weakened by cancer, AIDS, or certain medicines have an increased risk for serious fungal infections. In these persons, a fungus can spread throughout the body, even to the brain, and be deadly.


Like bacteria, protozoa are one-celled organisms and are too small to be seen without a microscope. Because they love moisture, protozoa are most commonly found in the earth's fresh water, oceans, and soil. Only a few of the 30,000 protozoan species cause disease in humans; they include the species that cause amebiasis (from amoebas), giardiasis, and malaria. Although disease-causing protozoa usually spread through contaminated water, many can also be carried on contaminated food and dirty hands. Malaria is spread through the bite of a mosquito that injects the protozoa called Plasmodium into the blood.

Treating Infections Caused by Germs

When scientists look for new drugs to fight infections, they target specific vulnerable areas in a particular germ's structure or internal chemistry. Then they try to destroy or disrupt these vulnerable areas by using chemical medicines that are safe for humans. Because the different types of germs - bacteria, viruses, fungi, and protozoa - are so different in structure and function, the need to "aim" drugs at their specific vulnerable areas means that all medicines won't necessarily kill all types of germs. For example, the penicillin family of antibiotics kills bacteria by disrupting a component of their cell wall. Yet these same penicillins are useless against viruses, because viruses have no cell walls. The antiviral drug zidovudine (AZT) works by interfering with the mechanism of virus replication inside human cells, so it is useless against bacteria and other germs that don't reproduce by this form of replication.

For doctors, the first challenge in treating any infection is to quickly identify which type of germ is causing it. If it is determined that medication is necessary, the next task is matching the germ with the specific medicine that will attack it most effectively. Not all infections can be treated with medication. But when a successful match is made between the germ and medication, the patient needs to take the medicine as directed, for as long as prescribed, to make sure that all germs have been destroyed.


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