The effects of ecological factors (prey, competitors, predators and disease) and intervention (immobilization, radio-collaring, and vaccination) on population size and demography were investigated in Serengeti wild dogs (Lycaon pictus), an endangered canid, between 1965 and 1991. Variation in ecological factors explained most changes in demography, but did not explain a decline in adult longevity. A significant reduction in pack life and individual longevity was coincident with the introduction of routine intervention and consistent with pathogen-induced mortality. Survival varied significantly between categories of intervention, and between individuals likely to have been exposed to different degrees of social stress before intervention. The loss of all study packs in 1991 contrasted with the persistence of breeding packs outside the study area. The cause of the demise of most study packs is unknown. Monte Carlo simulations demonstrated that population extinction was unlikely to be the consequence of chance events alone. One explanation compatible with the evidence is an outbreak of viral disease induced by stress, possibly caused by intervention.