The MMR vaccine (mumps, measles and rubella) is an early vaccine given to infants to prevent several horrible and life threatening diseases. In a 1998 paper Andrew Wakefield fingered it was a cause of autism, and between 2000 and 2004 managed to create a media buzz over it. His work was later completely discredited (not that it had much credit to begin with) and found to be unethical, eventually losing his license to practice medicine because over the debacle. Despite the attention lavished on Wakefield's claim, however, several massive cohort studies have been available showing no risk of MMR leading to autism, including one submitted to the New England Journal of Medicine looking at all children born in Denmark, totalling 537,303 children and showing absolutely no evidence of a link. The main reason to cite this large value for N is that Wakefield's paper only looked at 13 children to "prove" the link. This was available in 2002, the height of Wakefield's media scare.
However, the damage from the research and the subsequent PR rampage from anti-vaccination groups cause a period of low vaccine compliance in England as parents avoided giving their children life saving medical care. This has been linked as the reason behind several major outbreaks of mumps.