Chapter Overview

This chapter focuses on the interactions of the human body with microorganisms. It begins by discussing the normal microbiota-those microorganisms that have established residence in or on the body. The relationship between humans and their normal microbiota is usually either mutualistic or commensal. On occasion, the interaction can shift to parasitism (a pathogenic relationship). In addition, microorganisms that are not part of the normal microbiota can be pathogenic. Humans resist parasitic relationships by employing both nonspecific and specific mechanisms. The nonspecific resistance mechanisms are explored in this chapter.