[vatueil]

I don't think it's the most likely explanation, far from it, but it wouldn't have to be a bioweapon to have been released from a lab.

The reemergence of H1N1 in 1977 is believed to have been the result of a lab accident or vaccine trial gone awry, for example: http://www.virology.ws/2009/03/02/origin-of-current-influenz...