Thirteen years ago, a deadly strain of avian flu known as H5N1 was tearing through Asia's bird populations. In January 2004, Chinese scientists reported that pigs too had become infected with the virus—an alarming development, since pigs are susceptible to human viruses and could potentially act as a "mixing vessel" that would allow the virus to jump to humans. "Urgent attention should be paid to the pandemic preparedness of these two subtypes of influenza," the scientists wrote in their study.