Microorganisms that establish infections in humans do so by a number of means. For example, some bacteria remain associated with the surface of host cells, but elaborate a coating that provides protection from host immune defenses and external antimicrobial agents, such as antibiotics. Another strategy that is used by a number of disease-causing bacteria, virtually all viruses, single-celled eukaryotic parasites, and protozoa is the invasion of the host cells to which the microorganisms adhere. Once inside the host cell, the invading microbe is shielded from host defenses and therapeutic antimicrobial compounds.

Following the invasion of host cells, the microorganisms can establish an infection inside the host cells. This is referred to as intracellular infection. Once again, by remaining inside the host, the bacteria or protozoa are shielded from attack.

Intracellular infection presents a problem for the host, since the infection cannot be...