In November, 2-year-old Mac became the sixth endangered Asian elephant born or kept at the Houston Zoo to die of elephant herpes since 2000. The precocious Mac had become one of the zoo's most popular animals, entertaining visitors by dancing, playing windpipes and standing on his head. His illness came on suddenly and vets, who know little about the virus other than that it is not sexually transmitted, were unable to save him. News of his death prompted People for the Ethical Treatment of Animals to call for an end to the zoo's elephant breeding program. But zoo officials responded that the virus is latent in elephants in the wild as well as in captivity and that its researchers were helping search for a cure.