The effects of hormones on sex differences in infection: from genes to behavior

Males of many species are more susceptible than females to infections cause d by parasites, fungi, bacteria, and viruses. One proximate cause of sex di fferences in infection is differences in endocrine-immune interactions. Spe cifically, males may be more susceptible to infection than females because sex steroids, specifically androgens in males and estrogens in females, mod ulate several aspects of host immunity. It is, however, becoming increasing ly more apparent that in addition to affecting host immunity, sex steroid h ormones alter genes and behaviors that influence susceptibility and resista nce to infection. Thus, males may be more susceptible to infection than fem ales not only because androgens reduce immunocompetence, but because sex st eroid hormones affect disease resistance genes and behaviors that make male s more susceptible to infection. Consideration of the cumulative effects of sex steroid hormones on susceptibility to infection may serve to clarify c urrent discrepancies in the literature and offer alternative hypotheses to the view that sex steroid hormones only alter susceptibility to infection v ia changes in host immune function. (C) 2000 Elsevier Science Ltd. All righ ts reserved.