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Agent host environment interaction

Infectious diseases are usually classified by agent on the basis of biological features (categories of metozoa, protozoa, bacteria, fungi, rickettsia, viruses) (Benenson, 1990). The agent enters the human host through portals such as the respiratory tract, gastrointestinal tract, genitourinary tract, conjunctiva, or skin and tissue. Studies in intensive care units (ICUs) have shown that Gram-negative colonization of tracheal aspirate, mouth, peri-anal skin or urine significantly increases the morbidity and mortality of ICU patients (Shekelle et al, 1999).
The interval between the time of agent contact with the host and the onset of disease (incubation period) is generally thought to be the time required for agent multiplication to a threshold point where the agent population is large enough to produce symptoms. The incubation period is largely dependent on the rate of growth of the agent in the host (Benenson, 1990).