although one can count virions using an electron microscope(Figure 9.4c and9.5a),the number of virions in a suspensioncan be more easily quantified by measuring their effects on the host.Using such a method,we see that a virus infectious unit is the smallest unit that causes a detectable effect when added to a susceptible host.This can be as few as one viron,although a larger inoculum is more often required.By determining the number of infectious units per volume of fluid,a measure of virus quatity,called a titer,can be obtained.