The World Health Organization has warned that mammalian outbreaks of bird flu could facilitate transmission to humans, as avian influenza viruses adapt to infect animals biologically closer to humans. Some mammals may act as “mixing vessels” for influenza viruses, leading to the emergence of new viruses that could cause harm to animals and humans, the WHO said. Rising numbers of H5N1 avian influenza detections among mammals have led to concerns the virus may find it easier to infect humans.