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 as a cause of autism, and between 2000 and 2004 managed to create a media buzz over it. His work was later completely discredited (though the medical community at large never accepted it to begin with) and found to be unethical, eventually losing his license to practice medicine over the debacle. Despite the media attention on Wakefield's claim, several large cohort studies have been available showing no risk of MMR leading to autism. This included one submitted to the New England Journal of Medicine which studied all children born in Denmark from 1991 to 1998, totaling 537,303 children, and found no evidence of a link. The main reason to cite so large a value for N is that Wakefield's paper only looked at 13 children to "prove" the link. The Denmark study was published in 2002, the height of the media scare.
Damage from the research and subsequent PR rampage from anti-vaccination groups cause a period of low vaccine compliance in England as parents avoided or delayed (for instance, refusing to give three vaccines at once due to fears of "overloading the immune system") giving their children life saving medical care. This has been linked as the reason behind several major outbreaks of mumps.