For the experiments considered here, the infection rate per virus, b, and the viral production rate, p, can be combined into a single parameter, leaving only three parameters to be determined. The rationale for this simplification is the fact that, during an infection, the two parameters play equivalent roles: doubling the rate at which virus is produced by cells will have the same effect on new infections as doubling the rate at which virus infects cells. Therefore, the only identifiable quantity is the product of the two rates, pb. Since their product has units of inverse time squared, we have chosen to express this quantity as a new characteristic time, the infecting time, t inf~ffi ffiffiffiffiffiffiffiffiffiffiffiffiffi 2= pb ð Þ p , which corresponds to the average time it takes a single virus-producing cell to cause the latent infection of one more (see Materials and Methods).