ABSTRACT
Introduction The biological factors involved in infection (infestation in the case of parasites) are:
At its most basic level, infection represents an adverse shift of balance in favour of microbial invasion that overwhelms the host defence system. In immunologically competent individuals the invasion is determined by the product of the virulence and the size of the inoculum of the pathogenic organisms. In immunocompromised patients low-virulence organisms and even commensals that are not normally considered pathogenic can obtain a foothold and cause serious infections.
