Resistance to disease depends on the general health of the individual and the proper functioning of the organism's systems. The human will be the organism used in this unit on body defenses. My apologies to the animal and plant technologies students who wanted a more detailed study of the body defenses involved with the cow and/or wheat. Many of the principles used by the human to fight disease are similar to the principles used by other organisms. In this unit on body defenses we will discuss the lines of defense, homeostatic activities that help maintain good health, signs and symptoms of disease, emerging infections, and common human pathogens.

Every organism constantly scouts its tissues for foreign and potentially threatening microbes. This searching is called surveillance. Our ability to defend against microbes is related to nutrition, fatigue, age, sex, and climate. The ability of a microbe to cause disease in the various species of living organisms is extremely variable. Humans do not get hog cholera; cattle are very susceptible to anthrax and birds are very resistant to anthrax; humans get gonorrhea and cattle do not; and opossums rarely get rabies which is very common in raccoons and skunks. Resistance to certain microbes and the diseases caused by these microbes isan inborn characteristic of some species. Diseases of cold-blooded animals (poikelotherms) are rarely transmitted to warm-blooded animals (homeotherms). It is thought that basic physiological and anatomical differences are important in lending resistance to certain microbes. Bacilli that cause anthrax do not survive the 45 degrees centigrade temperature of birds; and this is one of the simpler explanations of species resistance.

There is also evidence of variation in resistance to pathogens among the human races and ethnic groups. Blacks affected with sickle-cell anemia do not suffer from malaria because it is assumed that the parasite (Plasmodium) can not penetrate the distorted red blood cells of humans who have sickle-cell anemia. The Spanish conquest of the Aztec population in Mexico during the 1500s was probably due, not to the superior Spanish army, but to the fact that the Spanich soldiers were immune and carrying the pathogens that caused ourbreaks of smallpox, measles, typhoid fever, and influenza. Recent studies have also shown variation in susceptibility to TB and leprosy among ethnic groups.

Infectious diseases are more common in the very young and the elderly. Before microbes have colonized the infant, microbes like E.coli and Pseudomonas frequently cause diarrhea in these young children. Also people over 65 are more likely to get influenza probably because of reduced ability to defend against the pathogenic microbe. Also it is believed that the tendency for the prostate gland to enlarge in "older" men which causes difficulty in urinating may cause an increase in urinary tract infections. The variation in stress that a person encounters may change the level of the hormone cortisone. Cortisone has an anti-inflammatory effect and removes one of the body's defenses against pathogens. Salmonella infections are more common in laboratory animals experiencing stress. The relationship between food/famine and disease has been observed. Cholera is more common in undernourished individuals; however, the number of Vibrio required to cause cholera is greatly reduced when ingested with food because the food neutralizes the stomach acids that destroy the pathogen. Also continually overeating may cause an increase in Clostridium in the digestive tract. It is also believed that the lack of sucrose in the diet reduces the growth of Streptococcus in the mouth and shows a reduction in tooth decay.