According to the record of human suffering and death caused by smallpox, cholera, typhus, dysentery, malaria, and others. Establishes the eminence of the infectious diseases. Despite the outstanding successes in control afforded by improved sanitation, immunization, and antimicrobial therapy, the infectious diseases continue to be a common and significant problem of modern medicine. The most common disease of mankind, the common cold, is an infectious disease, as is the feared modern disease AIDS and Some chronic neurological diseases. There is a little doubt that the future will continue to reveal the infectious diseases as major medical problems.
Biological criteria of normality are based on statistical concepts. Body height may be used as an example. If the heights of every individual in a large sample were plotted on a graph, the many points would fall on a bell-shaped curve. At one end of the curve would be the very short people and at the other extreme the few very tall people. The majority of the points of the sample population would fall on the dome of the bell-shaped curve. At the peak of the dome would be those individuals whose height approaches the average of all the heights. Scientists use curves in determining what they call normal criteria. By accepted statistical criteria, 95 percent of the population measured would be included in the normal range—that is, 47.5 percent above and 47.5 percent below the mean at the very center of the bell. Looked at in another way, in any given normal biological distribution 5 percent will be considered outside the normal range. Thus the 7-foot (213-cm) basketball player would be considered abnormally tall, but that which is abnormal must be distinguished from that which represents disease. The basketball player might be abnormally tall but still, have excellent health. Thus, in any statistical analysis of health, the possibility of biological variation must be recognized.