Three types of microbes found and related to epidemiology include transcients, normal flora, and pathogens. The transcients are the microbes you constantly encounter during routine contact with items in your environment (car, books, clothing, friends). These microbes do not establish and grow in you; and handwashing and bathing removes transcient microbes.

A second type of microbes become residents or normal flora. Sites with normal flora include skin, upper breathing tract, digestive tract, urethra, vagina, and external ears. Some body fluids like blood, urine, semen, amniotic fluid, and saliva should not contain microbes prior to emission; likewise the heart, liver and brain should be free of microbes. The development of normal flora is called colonization, and you began to develop your normal flora during the birth process. Beginning with your parents, siblings, doctor, and friends (now college acquaintances), you have become inhabited by non-disease-causing microbes. Even breast milk and bottled mild were involved in determining your normal flora.

During our life, we have contracted disease causing microbes called pathogens. The number of microbes necessary to make you develop disease symptoms varies. The ID (infectious dose) varies from 1 Coxiella (Q-Fever) or Shigella (enteritis); 10 rabies viruses, Francesella (tularemia), and Mycobacterium (TB); 1000 Neisseria (gonorrhea); and 1 billion Vibrio (cholera). For a microbe to establish in a host, the microbe often has pili for attachment, a capsule to protect against phagocytosis, enzymes like mucinase, hyaluronidase, or keratinase to permit spreading; and toxins (poisons) to inhibit capillary and nerve cell activities. Thank goodness most microbes do not survive and multiply in our bodies.