Ro and Rp

The 'reproductive rate' of an infection in a population - the number of new individuals infected by one infectious individual.  R_o is a theoretical value based on a large, non-immune, host population, while R_p (or some other subscript) is a measure of the real reproductive rate in a given population - and may change over time.  When the R_p is <1, the infection will die out (incidence will decrease) over time.  When R_p =1, the infection will be present at the same incidence and is said to be endemic.  When R_p >>1, the infection will spread rapidly, the incidence will rise and the infection is said to be epidemic.