you know nanner is pretty accurate on this one. the big fear is that the virus will mutate into a human virus then away we go, cause no one will have any natural resistance to it. now the chances of it mutating are very small, and thats why nanner is right. literally what needs to happen is for two virus's to infect the same cell. one avian flu, the other some human airborn virus. so they infect the same cell at the same time. then by shear luck there RNA gets mixed up. and the part of the RNA in the human strain that involves infecting people gets mixed in with the avian flu genome. after all that happens, the few viruses that get this combination much survive to get into another cell(once again very hard, there is a reason why viruses have to make 10000 of themselves after each replication cycle.) once it infect the new cell it needs to successfully replicate itself, the new RNA combination might nto be complete and the virus could be a dud. and there you have it all the unlikely events that have to happen before we get the killer flu. and thats a VERY genral overview.