The Measles Outbreak: In December 2014, a measles outbreak originated in Disneyland, resulting in 133 cases of the disease, including 40 in visitors to Disneyland between December 17 and 20. Most of these were unvaccinated children. The likely patient zero would have been an international traveler visiting the park from a country currently experiencing an outbreak. In total, at least 127 cases of measles have been directly traced to the Disneyland outbreak.
Seriously though I am surprised deaths don't happen more often. The Walt Disney parks had 132,549,000 visitors just in 2013. So just in that year alone the average was more than 37 thousand people per day.
I'll bet you it is statistically safer to play and have fun inside their parks than the trip flying or driving there.