Why Bird Viruses and Human Viruses Increase Virulence in Winter

Testosterone is an immunosuppressant. Testosterone reduces both cell-mediated and humoral immunity in birds (Behvioral Ecology 2000; 11: 654-662). In the "Northern pintail duck," testosterone is lowest from July-September, increasing thereafter throughout the winter to reach maximum levels during the breeding season of April-June (Reproduction, Fertility and Development 2000; 12(4) 229 - 235).

It is my hypothesis that DHEA is necessary for immune function. A number of studies have demonstrated the effectiveness of DHEA against viral and other infectious agents. Testosterone reduces conversion of DHEAS, the background source of DHEA, to DHEA. Since DHEA may be involving in heat production, cold weather should increase use of DHEA for heat. It is known that avian viruses are increased by increased cold. Increasing levels of testosterone during winter could explain why avian viruses increase during the winter season. This could also explain why humans exhibit increased vulnerability to viruses in the winter since testosterone increases in autumn and winter in humans.

It is my hypothesis that the "secular trend," the increase in size and earlier puberty in children, is caused by an increase in the percentage of individuals of higher testosterone within the population with time. This is a periodic trend because excessive testosterone increases morbidity and mortality which eventually reduces the percentage of individuals of high testosterone within the population. I suggest we are currently in a period of increased testosterone which has allowed viruses to enter humanity, for example, the HIV.

DHEA is highest around age twenty. Periods of very low DHEA occur in the very young and the elderly; these are ages more affected by viruses.