West Nile may be fatal years after infection and recovery, according to a new study. © Reuters

Since West Nile fever first appeared in the United States in 1999, more than 45,000 people have been infected, and nearly 2000 of them are known to have died, for a roughly 4% fatality rate. But a new study, reported today here at the annual meeting of the American Society of Tropical Medicine and Hygiene, suggests that the fatality rate may be much higher. That’s because people infected with the virus may still die years after recovery, because it leaves them more vulnerable to other infectious diseases and kidney problems like renal failure. “This is exciting work,” says Gregory Ebel, a virologist at Colorado State University in Fort Collins, who was not involved in the study. “It’s really strong support for a case that has been building for almost 10 years now.”

West Nile fever was long thought to be a manageable public health problem. The mosquito-borne virus—a relative of the Zika and yellow fever viruses—causes symptoms in just one out of every five infected people. Most of those experience fever and a flulike illness. But in some patients, the virus also infects the central nervous system, which can be fatal. Patients who survive tend to do worse later in life, according to some anecdotes. What’s more, two small studies, one in Israel in 2005 and one in Colorado in 2012 also suggested that overall mortality might be higher than acute cases suggest.