© 2002 British Ecological Society

1. The effect of cowpox virus on survival in two rodent hosts was investigated using nearly 4 years of longitudinal data from two sites. 2. We investigated whether an individual’s probability of infection influenced the prob-ability of surviving the next month. We also investigated the effect at the population level, examining whether, in addition to seasonal effects, changes in cowpox prevalence explained further temporal variation in survival rates. 3. In bank voles, but not wood mice, individuals with high probabilities of infection survived better than uninfected animals. 4. At the level of the population, the effect of infection on survival varied through the year in both species. Survival rates in late summer increased with cowpox prevalence, whilst survival rates in winter decreased with cowpox prevalence. 5. We discuss why parasites such as cowpox virus may increase or decrease host survival and why the effect may depend on the time of year. Key-words: Apodemus sylvaticus, capture–mark–recapture, Clethrionomys glareolus, endemic infection, pathogen