Hypothesis: increased male mortality caused by infection is due to a decrease in heterozygous loci as a result of a single X chromosome.

Inbreeding in experimental animals leads to loss of heterozygous loci and a marked increase in morbidity and mortality. Males have fewer heterozygous loci than females because of a single X chromosome. It is suggested that heterozygous loci protect against infection and that increased male mortality in humans at all ages is secondary to infection. The specific testable hypothesis is that episodes of bacteraemia occur throughout life leading to toxin secretion causing sudden death in infancy (SUDI), accelerating the development of atherosclerosis and precipitating sudden death in old age.