Many diseases are most contagious in the prodromal period because of increased viral numbers and the lack of a clear disease state. This confers an advantage to the organism, because it maximizes the spread of disease before a quarantine can be effective. In reality, contagion of the disease can happen at any point in the illness process. Different diseases will be contagious at different times during the infection. There are several types of infectious diseases. In an acute disease, the changes are relatively sudden, with active disease symptoms and short incubation periods. Influenza is generally an acute disease. Chronic diseases can last months or years and may never clear out completely. This involves a prolonged period of infectivity and communicability. Hepatitis C and Helicobacter pylori cause chronic infectious diseases. In latent diseases, the pathogen becomes dormant and is not transmissible. This is what’s seen with mononucleosis, caused by Epstein-Barr virus, herpes simplex viruses, and chickenpox viruses. The reactivation can happen many years later.
PATHOGENS The understanding of pathogens was advanced by the work of Robert Koch, who developed Koch’s postulates. It should be noted that Koch’s postulates do not work in all cases of infectious diseases but are good general rules to follow. According to these postulates, an infectious disease must follow these rules: 1. The pathogen must be present in every person who has the disease but not in healthy persons. 2. The pathogen should be able to be isolated and grown in a pure culture. 3. A healthy person who becomes infected should develop the same symptoms as the sick person. 4. The pathogen should be isolated from the new host and must be the same as in the original host. There are limitations to the postulates. Examples include Helicobacter pylori, which can be present in the healthy person and in the person with disease. In addition, not all healthy subjects can develop the disease when exposed. There are host factors that prevent getting the disease in certain resistant hosts. In addition, two different people
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