ABSTRACT
The human body has several successive lines of defense, all of which must be overcome if a pathogen is to establish a long-term infection in a human host. As described in Chapter 2, when a pathogen breaches one of the protective epithelial surfaces of the body and gains access to the soft internal tissues, it encounters a standing army in the form of the immediate defenses of innate immunity. If these cellular and molecular defences are successful in repelling the pathogen within a few hours, the effects of the infection, and of the immune response against it, are small, and may not even be perceived by the human host. Most people have these minor skirmishes with microorganisms on a daily basis, particularly in childhood. This chapter considers what happens if the pathogen is able to outrun the immediate defenses of innate immunity, simply by expanding its population at a greater rate than the defenses can kill the pathogens or eliminate them from the human body.
