Flu: The Story of the Great Influenza Pandemic of 1918 and the Search for the Virus That Caused It

by Gina Kolata

Webster proposed that the worst flu pandemics, the one in 1918 being at the far end of bad, start with a bird flu. But before it can infect a person, it has to be humanized—that is, to change in a way that would allow it to keep the birdlike features that make it so infectious and yet acquire human flu-like properties that would allow it to grow in the lung cells of a human being. That crucial step, Webster said, typically takes place in pigs. Pigs bridge the gap between birds and humans—both bird flu strains and human flu strains can grow in pigs’ bodies. An unfortunate pig that happens to be infected with both a bird and human virus at the same time can become a mixing bowl, with the genes from the two types of flu viruses recombining in its cells to form a new hybrid virus that can infect humans but has some genes from the bird flu, genes that make the newly emerging virus more dangerous than any that had been around before. Thus the stage can be set for a worldwide pandemic.