1. definitions Viruses are obligate intracellular parasites which can only be viewed with the aid of an electron microscope They vary in size from approximately 20-200 nm. In order to persist in the environment they must be capable of being passed from host to host and of infecting and replicating in susceptible host cellsViruses are obligate intracellular parasites which can only be viewed with the aid of an electron microscope. They vary in size from approximately 20-200 nm. In order to persist in the environment they must be capable of being passed from host to host and of infecting and replicating in susceptible host cells. 1. definitions