All told, just shy of 3,000 people in the U.S. have died of West Nile since 1999. But experts warn of the risks of underestimating the virus. Although most infections are asymptomatic, the virus triggers a fever in about 20 percent of people, and a small proportion of cases—about one in 150—are what doctors call “neuroinvasive,” meaning they reach the brain, triggering encephalitis, or the spinal cord, triggering meningitis.

In either scenario, neuroinvasive West Nile can lead to serious consequences. People can develop a stiff neck and devastating headache. “They’ll say it’s the worst headache they’ve had in their entire life.

And because there’s no specialized treatment for the disease, doctors can only treat the symptoms and hope for the best. Many people with a neuroinvasive infection never fully recover.

Our meningitis cases, about 40 percent of them never returned back to their baseline status, and for our encephalitis cases, [that figure] was 80 percent.