Humans occasionally pick up influenza viruses from pigs. Reports of human-to-pig transmission are apparently RARE, but such cases are assumed to happen. The tentative detection of the novel swine influenza H1N1 virus in an Alberta swine herd over the weekend shook Canada's pork industry and raised concern about the potential for new hybrid viruses to emerge. Canadian authorities said on May 2 that preliminary testing detected the virus in an Alberta herd and that it probably came from a Canadian carpenter who works on the farm and had a flu-like illness when he returned from a visit to Mexico in mid-April. The worker had contact with the pigs on Apr 14 and about 220 pigs in the herd of 2,200 began showing signs of sickness on Apr 24. The carpenter has recovered and the pigs were recovering. Pigs are often infected with flu viruses, including strains from humans and birds. They are described as a mixing vessel where different viruses can trade genes (reassort) and produce new variants. The novel H1N1 virus itself has been said to contain genetic material from swine, avian, and human flu viruses. The virus isolated from the swine does not appear to differ from the virus spreading among humans. "There is no sign that it has changed at all. But this could of course happen like with any other flu viruses." Despite repeated official assurances that proper cooking destroys any flu viruses in pork, ten countries have banned Canadian pork products since the Alberta finding. The USDA said it is "actively working to develop an H1N1 vaccine for swine, just as the CDC is doing for humans."