New research may explain why some inflammatory diseases are more common in the winter. According to Todd and colleagues, the activity of ∼23% of human genes varies with the seasons. Genome-wide analyses of mRNA levels in peripheral blood mononuclear cells from European cohorts revealed a more inflammatory status in the winter compared with in the summer months. For example, genes involved in B cell receptor signalling and metabolic processes were more highly expressed in the winter, as were the pro-inflammatory factors soluble interleukin-6 receptor and C-reactive protein. In The Gambia in West Africa, the heightened inflammatory status correlated with the rainy season, from June to October, which is when infectious diseases peak in that region. So, seasonal gene regulation may provide readiness for better protection from infections, but it may also contribute to the higher frequencies of heart attacks, rheumatoid arthritis and type 1 diabetes that are observed in the winter months.