New human cases of avian influenza may result from two alternative processes: i) infection of humans from animal sources [4] , or ii) human to human transmission [23] . For a standard epidemic, explicit consideration of multiple introductions is not important as each case produces many secondary infections. For emerging infectious diseases multiple introductions from a reservoir [22, 25] may constitute an important fraction of all observed cases, and the progression of secondary cases must be carefully assessed and monitored.