ABSTRACT
This chapter gives an overview of the different kinds of pathogens, as well as those microorganisms that colonize our body without causing trouble. It then discusses the cell biology of infection—the molecular interactions between pathogens and their host. Some viral and bacterial pathogens can cause or contribute to chronic, life-threatening illnesses that are not normally classified as infectious diseases. The human body contains about 1013 human cells, as well as a microbiota consisting of approximately 1014 bacterial, fungal, and protozoan cells, which represent thousands of microbial species—the so-called normal flora. Infectious diseases are caused by pathogens, which include viruses, bacteria, and fungi, as well as protozoan and metazoan parasites. All pathogens must have mechanisms for entering their host and for evading immediate destruction by the host. Eukaryotic pathogens such as fungi and protozoan parasites typically pass through several different forms during the course of infection.
