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Chapter 20 : Infection and Immunity
Pathogenic microbes gain entrance to the body in characteristic ways, depending on the microbe. The portal of entry for the bacteria that cause typhoid and paratyphoid fevers, dysentery, and cholera is the digestive tract. An important aspect of defense against microbial infection is the activity of special phagocytic cells that are very mobile and can engulf and destroy microbes. Individuals with phagocytes that do not function properly may have recurrent infections caused by microbes that are normally not pathogenic, and some forms of "phagocyte cell disease" are often fatal in childhood. The human immunodeficiency virus (HIV) attacks the host’s immune system, causing its eventual failure. This failure leaves affected individuals vulnerable to many infections and cancers, leading inexorably to severe morbidity and high mortality. Substantial evidence suggests that HIV emerged in the middle of the twentieth century, following the infection of humans with simian immunodeficiency viruses. Disparities in risk of infection and in access to treatment expose critical inequities in the distribution of social and medical resources within developed and developing countries. Theodore Rosebury made extensive studies on the aspect of microbiology and summarizes the overall microbial ecology of humans. Germ-free experimental animals become very ill, and may even die, if their diets are not supplemented with certain vitamins that do not have to be added to the diets of normal animals. This indicates that in normal animals, the microbial population in the intestine must furnish vitamins to the host.