As William Trager pointed out some time ago in a review in Science, all obligate intracellular parasites—be they viral, bacterial, or protozoan—face a common dilemma. That dilemma is to invade their host cells in a way that is not destructive of the host cell upon whose metabolic hospitality and functional well-being their own reproduction depends. Simply stated, these organisms must penetrate the plasma membranes of their host and take up residence in a suitable location in the cell's cytoplasm. Since many of the speakers in this session will address themselves to the issues of penetration and intracellular location of specific organisms, I view my task as one of trying to place these issues into a general conceptual framework.

There are three general paths an intracellular parasite might follow to gain entry into an animal cell (Fig. 1): a) direct passage of the parasite through the host cell's plasma membrane; b) fusion of the outer membrane of the parasite with the cell's plasma membrane; c) endocytosis of the parasite by the host cell.