LOSS OF APPETITE Experiments show that helpful gut bacteria like this E. coli K12 produce proteins that could influence the appetite of mice and rats.
Kwangshin Kim/Science Source
EXTRA BABIES Roundworms may act as a fertility aid: Tsimane women in Bolivia infected with the intestinal parasites may have more children than uninfected women.
ESSENTIAL ELEMENT Brains and testes both require selenium, a nutrient people get from such foods as fish, organ meats and plants grown in selenium-rich soil. Mouse studies show that when the element is in short supply, the testes and brain compete.