Koch's Postulates Koch's studies with bacteria led him to develop rules for identifying the microorganism that causes a specific disease. These rules are known as Koch's postulates.

1. The pathogen must always be found in the body of a sick organism and should not be found in a healthy one.

2. The pathogen must be isolated and grown in the laboratory in pure culture.

3. When the cultured pathogens are introduced into a healthy host, they should cause the same disease that infected the original host.

4. The injected pathogen must be isolated from the second host. It should be identical to the original pathogen.

Koch's ideas played such a vital role in the development of modern medicine that he was awarded a Nobel Prize in 1905. Today, we know that there can be exceptions to these rules, but they remain important guidelines for identifying the causes of new and emerging diseases.

Symbionts vs. Pathogens Parts of the human body provide excellent habitats for microorganisms. Fortunately, most microorganisms that take advantage of our hospitality are symbionts that are either harmless or actually beneficial. Yeast and bacteria grow in the mouth and throat without causing trouble. Bacteria in the large intestine help with digestion and produce vitamins. In fact, if all your cells disappeared, the outlines of your body and digestive tract would still be recognizable—as a ghostly outline of microorganisms!

What's the difference between harmless microorganisms and pathogens that cause disease? The “good guys” obtain nutrients, grow, and reproduce without disturbing normal body functions. The “bad guys” cause problems in various ways. Some viruses and bacteria directly destroy the cells of their host. Other bacteria and single-celled parasites release poisons that kill the host's cells or interfere with their normal functions. Parasitic worms may block blood flow through blood vessels or organs, take up the host's nutrients, or disrupt other body functions.

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