Virologist Robert Webster thinks that the H5N1 strain of the avian influenza virus poses the most serious public health threat since the Spanish flu pandemic of 1918, which killed an estimated 40 million to 100 million people worldwide. Although the H5N1 strain has so far shown no signs that it has acquired the ability to transmit easily from person to person, Webster says that it is only a matter of time before it does. For that to happen, Webster and others believe that a version of the human flu virus, which is easily transmittable between people, and the H5N1 avian virus would have to infect the same mammalian cell at the same time and re-combine their DNA. If H5N1 picks up those genes from the human flu virus that enable it to spread from person to person, Webster says that virtually nobody will have immunity to it, and many deaths may ensue.