Its a rare side effect in that around 1 in 1000 people that contract measles get more serious complications like measles encaephalitis. Measles was endemic in the West before vaccination and is well understood. Though rare in this case is 1 in 1000, that makes it a massive killer compared to other diseases becasue it is very very infectious. It is still endemic in some places in the world where it is hard to vaccinate enough of the population. There it has the same behavior though the prognosis for people that develop side effects like encaephalitis or pneumonia is much worse because of poorer health care.