Given the highly focussed temporal occurrence of the clusters (appearing and disappearing over the timescale of a few weeks), it might be inferred that if a putative infectious agent is involved this is highly transmissible and produces acute infections that rapidly result in death or recovery and the development of resistance. It is likely such a pathogen could only be maintained by rapid spatial movement through the wider population, via locally unstable dynamics, or by inactive latent infections in some hosts acting as a reservoir and intermittent source of infection through time. One possibility consistent with some of these considerations is cowpox virus, infections with which can show fine-scale temporal clustering in wood mouse populations [36] . However, cowpox virus is known to suppress TNF-a responses [37] , which were expressed at very high levels in cluster 1 mice. Furthermore, field studies of cowpox in UK wild rodents [38, 39] generally indicate that infection levels within populations persist with a much coarser temporal grain than was the case for the tightly focussed episodes of infection recorded here.