Jenny McCarthy's crusade against routine childhood vaccinations, specifically MMR, began after her son was diagnosed with autism a short period of time after having been vaccinated. Her celebrity status has given her far more authority than she would have had just being an average mom, and despite having no medical training or background, people believe her rather crackpot pseudo-scientific stance.
The controversy largely revolves around recently discredited research from 1998, regarding a non-replicable link between MMR shots and autism. Even though the entire basis of the anti-vax crusade was literally torn out from under them, Jenny McCarthy has not retracted from her stance against childhood vaccination, demonstrating a close-minded disregard for the principle of scientific skepticism.
Despite the complete absence of evidence supporting their view, anti-vaccine proponents still cling to their pseudo-scientific view, and not only does this endanger their children who remain unvaccinated and die from diseases that have been largely kept under control for a century, the practice of non-vaccination endangers everyone, due to the fact that even just a small amount of people not being vaccinated threatens herd immunity for the entire population.