Analysis at the University of Wisconsin at Madison (UW) revealed that a key component of the immune system, a gene called RIG-1 appeared to be involved. Levels of the protein produced by the gene were lower in tissue infected with the 1918 virus, suggesting it had a method of switching it off, causing immune defences to run wild.

This ability to alter the body’s immune response is shared with the most recent candidate for mutation into a pandemic strain, the H5N1 avian flu. Experts are worried that if the virus changes so that it can infect humans easily, it could again be far more lethal than normal seasonal flu. “What we see with the 1918 virus in infected monkeys is also what we see with H5N1 viruses,” said Yoshihiro Kawaoka, who led the analysis at UW.