Both the plaque velocity and viral titer growth rate depend on the infection and production rates only through pb. Since this quantity has units of inverse time squared (units of virus cancel), it is useful to rewrite the dependence on these rates as a characteristic time, ffiffiffiffiffi ffi 2 pb s . A physical meaning can be ascribed to this quantity by considering Equation (2) in the case of a single infectious cell (I 0~1 ), within a completely susceptible cell population (T 0~N ). If loss of viral infectivity, c, is neglected, the equations can be then integrated to show that t inf~ffi ffiffiffiffi ffi 2 pb s is the time for that single infectious cell to cause the (latent) infection of one more cell. Therefore, we call this characteristic time the infecting time.