Four out of five of people infected with the mosquito-borne West Nile virus (WNV) won’t even know it—heartening news when you consider there’s no vaccine to prevent the disease nor targeted medications to treat it. However, the rest can develop a serious illness—particularly the approximately 1% who get encephalitis, an inflammation of the brain that requires hospitalization. Up to 20% of these patients die. What is it that makes these select few so vulnerable? An international team of researchers led by Rockefeller University’s Jean-Laurent Casanova, MD, PhD, and Alessandro Borghesi, MD, PhD, of the San Matteo Research Hospital in Pavia, Italy, recently pinpointed the cause: an autoimmune defect that prevents the body from defending itself.