Influenza A is one of two influenza viruses that fuel costly annual flu seasons and pose a near constant threat to humans and many other animals. It's also responsible for occasional pandemics that, like the one in 1918, leave millions dead and wreak havoc on health systems and wider society. Influenza A was first identified as a health threat nearly a century ago, but only in the last decade have scientists identified one of the virus’s key proteins for infiltrating host cells and short-circuiting their defenses. Now, a team of researchers at the University of Wisconsin–Madison and other institutions have taken a major step toward understanding how that protein works, with a new finding that runs counter to previous conventional wisdom: They’ve found that viruses like influenza A take over host cells in a way that more closely resembles a tactical strike than brute force. The protein in question is known as PA-X. It disrupts host cells by degrading their RNA--the genetic material cells need to make proteins for all sorts of purposes, including mounting a defense against invading viruses. PA-X makes thousands of cuts in the RNA of host cells, cleaving the cells' genetic handbook into an indecipherable mess.