Many parasites, including all known viruses, characteristically lead an intracellular existence, and are known as ‘obligatory intracellular parasites’. It is, however, obvious that—whether the parasite is a virus, a bacterium, or a protozoon— transmission of infection from host to host, or from cell to cell within a single host, must involve an extracellular stage, however short in duration. This extracellular stage is, for the purpose of therapeutics of highest importance, since evidence accumulates that within the protecting walls of a cell parasites are relatively protected against chemotherapeutic agents and antibiotics, while susceptible to their action in the extracellular phase.