Acute infections with highly pathogenic viruses, bacteria or protozoa are often associated with extreme expression of cytokines and TLRs [30, 31, 32, 33] . We discovered significant temporal clusters of individuals with extreme expression of immunologicallyrelevant genes, including cytokines and TLRs, in late June (cluster 1) and early September 2008 (cluster 2). This is consistent with the presence of an acute, highly immunogenic microbial infection in the mouse population at these times. Other possible causes for the ''extreme expression'' clusters are considered less likely. Thus, autoimmunity, allergy or malignancy not involving infection does not well explain the clustered temporal epidemiological pattern and poisoning is unlikely because the habitat is managed for pheasant shooting (pheasants would be likely to take poisoned bait placed for rodents) . The pattern seen is also difficult to attribute to handling stress or to ontogenetic or genetic variation. Compared to the non-cluster animals in the present study and to hundreds of other ''normal'' wild rodents (A. sylvaticus and Microtus agrestis) recently analysed in our laboratory in a similar way [16, 18] , the relative mRNA expression ratios for immunological genes and the amount of constitutive TNF-a protein secretion in some cluster animals are unprecedentedly high. The variation in expression between cluster and non-cluster animals also seems to be much greater than constitutive variation in expression reported amongst wild-derived Mus inbred laboratory lines [34, 35] .