This volume presents evidence that infection is cyclical with the seasons, and that this phenomenon is mirrored in cycles of immune function. The authors identify the mechanisms by which immune systems are bolstered to counteract seasonally-recurrent stressors, such as extreme temperature reductions and food shortages. Stress, infectious diseases, autoimmune diseases, and human cancers are examined, and the role of hormones such as melatonin and glucocorticoids are considered. This is the first monograph to examine seasonal immune function from an interdisciplinary perspective.