Scientists have made a troubling discovery about measles that may explain the deaths of 16 adults and children in California. They believe that a deadly measles complication that results in the death of kids years after they seem to recover may actually be way more common than we though.

That’s bad news as cases of measles begin to rise around the country. It’s a condition caused subacute sclerosing panencephalitis or SSPE, which is what happens when the measles virus stays in the brain for years even after the child has recovered, according to an Infectious Diseases Society of America statement.

Scientists don’t know why, but it apparently reawakens and wreaks havoc, resulting in seizures, coma and even death. There is no known treatment for it, and it has a frightening 100 percent mortality rate.

Scientists had thought this condition to be very rare, but this new study indicates that rather than infecting 1 in 100,000 kids, it may actually affect 1 in 600 people who get measles as infants.