CORBIS
The 1918 influenza pandemic caused an estimated 50 million to 100 million deaths worldwide.
The virus that caused the 1918 influenza pandemic probably sprang from North American domestic and wild birds, not from the mixing of human and swine viruses. A study published today in Nature1reconstructs the origins of influenza A virus and traces its evolution and flow through different animal hosts over two centuries.

“The methods we’ve been using for years and years, and which are crucial to figuring out the origins of gene sequences and the timing of those events, are all flawed,” says lead author Michael Worobey, an ecologist and evolutionary biologist at the University of Arizona in Tucson.
Worobey and his colleagues analysed more than 80,000 gene sequences from flu viruses isolated from humans, birds, horses, pigs and bats using a model they developed to map evolutionary relationships between viruses from different host species. The branched tree that resulted showed that the genes of the deadly 1918 pandemic virus are of avian origin.

Birds blamed

Birds have been implicated in the deadly strain’s origins before. A 2005 genetic analysis of the 1918 pandemic virus pulled from a victim’s preserved tissue concluded that it most closely matched viruses of avian origin2. But a 2009 study3found instead that the viral genes circulated in humans and swine for at least 2 to 15 years before the pandemic and combined to make the lethal virus.