Christine L. Case, Ed.D.
Biology Professor
Skyline College

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Introduction

Most microbes are beneficial to the ecosystem and do not cause disease. Many microbes are opportunistic pathogens, that is, they don’t normally have access to a host but if put in a host when the host is weakened from another disease or malnourished, disease results. People whose immune systems are weakened by AIDS or cancer chemotherapy can get infections such as Pneumocystis pneumonia that healthy people don’t get. Hospitalized patients and patients undergoing surgery can acquire infections during their medical stay. These are called nosocomial infections.

A few microbes that grow in and on a host almost always harm that host, these microbes are called parasites or pathogens. There is a wide range of pathogens from the Rhinoviruses to Vibrio cholerae to Mycobacterium tuberculosis. Rhinovirus causes the common colds. Colds are an example of an acute disease, that is, it lasts a short time. The host is able to walk around which ensures the virus can be transmitted to a new host. A mutant Rhinovirus that killed its host might not have time to be transmitted. Cholera also causes an acute and often fatal disease. The violent diarrhea of cholera ensures that the V. cholerae bacteria are out of the sick host so they might contact a new host. M. tuberculosis causes a long-term or chronic disease which will give the bacterium a host for a long time with adequate time for transmission.

The stages of disease follow a pattern although the length of time for each stage and the final outcome may vary with each disease. After contact between pathogen and host, the pathogen starts to grow during the incubation period. The length of incubation periods varies widely.The first disease symptoms appear during the prodromal period, this is followed by the period of illness. The period of illness is short for acute diseases and can be quite long for chronic diseases. The period of illness is followed by the period of decline and, usually, the period of convalescence.

Until the 1880s, people didn’t know what caused diseases. They knew diseases were associated with stagnant water, the miasma of which Darwin wrote in 1845. The association of disease with the mosquito-ridden slow-moving water gave rise to the name of one of those diseases–malaria is from Italian for "bad air." Pasteur believed there was a relationship between microorganisms and disease because he has proven that microorganisms caused "diseases" or spoilage of food. Both Pasteur and Agostino Bassi had observed microorganisms growing on diseased silkworms. However, it isn’t enough to see the microorganism on the diseased host; the microorganism could be the result of the disease and not the cause. It was Robert Koch who provided an procedure to prove that a specific microorganism causes a particular disease. We still use his procedure, called Koch’s Postulates, to determine the cause of new diseases. In recent years, Koch’s Postulates have been employed to discover the cause of several diseases including Lyme disease, ulcers, and AIDS.

With the advent of antibiotics, vaccinations, and sanitation, people living in the developed countries believed that infectious diseases would be a thing of the past. However, modern communications and new techniques for identifying microbes have made us more aware of emerging infectious diseases in recent years. The study of disease transmission and distribution is epidemiology. Epidemiologists monitor the incidence and prevalence of diseases. The central source of epidemiologic data in the United States is the Centers for Disease Control and Prevention (CDC). The CDC works with health agencies around the world to monitor the spread of diseases.