Host–parasite relationship is determined by the interaction between host factors and the infecting microorganisms. Outcome of any microbial infection depends on the interaction between the host and the parasite. The relationship of existence between the host and parasite may be (a) symbiosis, (b) com-mensalism, or (c) disease process:
Symbiosis describes a situation where both the microorganisms and host species live together with mutual benefit. There is an element of symbiosis in the relationship between the human host and the gut flora; humans provide the bacteria with a warm, moist environment for their survival and gut flora provides a natural barrier against many invading pathogens.
Commensalism is an association in which only the micro-organism derives benefit, without causing any injury to the host. Most human microbes are commensals. They are present as bacterial flora of the skin and mucous mem-branes, including the upper respiratory tract, the lower gastrointestinal tract, and the vagina.
Disease is caused by certain microorganisms known aspathogens. Microorganisms vary in their ability to causedisease in humans.