31.   The scientists know, from looking at genetic variants of human AIDS, that the virus infected humans on at least several occasions before becoming a full-fledged epidemic.

32.   The virus, which is present in rodent droppings, infects humans when inhaled into the lungs.

33.   The viral progeny might then infect humans nearby.

34.   There is no evidence that chronic wasting disease infects humans.

35.   Under the right conditions, natural selection may allow survival of those strains best suited to infect humans.

36.   Unlike mad cow disease, foot-and-mouth disease rarely infects humans, and meat from infected animals is safe to eat.

37.   When did the AIDS virus first infect humans?

38.   Whether humans were infected by bovine products, or from an independent source entirely, remains unknown.

39.   Will animal viruses carried in transplanted tissues be able to infect humans?

40.   Xenotransplant researchers say there is no evidence that they infect humans outside the laboratory.