Pennsylvania officials are reporting a fourth measles-associated death. An 18-year-old from Mifflin County died from a severe neurological complication of the virus. None of the four who died were vaccinated. The U.S. is experiencing its worst measles year since 1991, with seven deaths in two years. Pennsylvania's outbreak includes nearly 700 illnesses across 38 counties. These are the first measles-associated deaths in the state in 35 years. Health officials emphasize the importance of the MMR vaccine to prevent outbreaks. International health officials will meet in November to assess the U.S. and Mexico's measles-free status.