Germ Theory of Disease

Louis Pasteur along with Robert Koch developed the germ theory of disease which states that "a specific disease is caused by a specific type of microorganism."

In 1876, Robert Koch established an experimental procedure to prove the germ theory of disease. This scientific procedure is known as Koch's postulates.

Koch's postulates:

• the causative agent must be present in every case of the disease and must not be present in healthy animals.

• the pathogen must be isolated from the diseased host animal and must be grown in pure culture.

• the same disease must be produced when microbes from the pure culture are inoculated into healthy susceptible animals.

• the same pathogen must be recoverable once again from this artificially infected animal and it must be able to be grown in pure culture.

Koch’s postulates not only proved the germ theory but also gave a tremendous boost to the development of microbiology by stressing a laboratory culture and identification of microorganisms.

Circumstances under which Koch’s postulates do not easily apply:

• Many healthy people carry pathogens but do not exhibit the symptoms of disease. These "carriers" may transmit the pathogens to others who then may become diseased. Example: epidemics of certain hospital acquired (nosocomial) infections, gonorrhea, typhoid, pneumonia, and AIDS.

• Some microbes are very difficult to grow under in-vitro (in the laboratory) conditions. Example: viruses, chlamydia, rickettsias, and bacteria that cause leprosy and syphilis. Some of the fastidious organisms can now be grown in cultures of human or animal cells or in small animals.

• not all laboratory animals are susceptible to all pathogens. Many pathogens are species specific. Ethical considerations limit the use of laboratory animals and human volunteers.

• Certain diseases develop only when an opportunistic pathogen invades a susceptible host. These secondary invaders or opportunists cause disease only when a person is ill or recovering from another disease. For example in case of pneumonia and ear infections following influenza, isolation of bacteria causing pneumonia may mislead the isolation of influenza virus.

• Not all diseases are caused by microorganisms. Many diseases are caused by dietary deficiencies (scurvy, rickets). Some of the diseases are inherited or are caused by abnormality in chromosomes. Still others such as cancer of the lungs, and skin are influenced by environmental factors.

Source: http://www.towson.edu/~wubah/medmicro/Germ_theory.htm

For an alternative perspective on germ theory go to: http://www.unhinderedliving.com/germtheory.html

Which of these theories do you find most compelling? Why?