Epidemics of contagious prion diseases can be perpetuated by horizontal (animal to animal) and maternal (dam to offspring, before or after birth) transmission^,,,,,, but the relative importance of each mechanism is unclear. Here we compare the incidence of chronic wasting disease (CWD) in captive mule deer (Odocoileus hemionus) that is attributable to horizontal or maternal transmission. We find that horizontal transmission is remarkably efficient, producing a high incidence of disease (89%) in a cohort of deer in which maternal transmission was improbable. Our results indicate that horizontal transmission is likely to be important in sustaining CWD epidemics.