In 1998, then-gastroenterologist Andrew Wakefield (who's now barred from practicing medicine) published a fraudulent paper in the medical journal The Lancet purporting to show a link between autism and the measles, mumps, and rubella vaccine. The study has since be debunked, but for some reason, fringe anti-science groups cling to it, insisting that vaccines can cause autism. Three out of the four United States presidential candidates continue to pander to these people, but everything Hillary Clinton has said about vaccines indicate that she's the only presidential candidate who actually believes in science.
While some of the other candidates are outright anti-vaxxers, and others stay wishy-washy enough on the subject so that they don't lose votes from either side, Clinton's stance remains clear. She spearheaded the Childhood Immunization Initiative and the Vaccines for Children program in 1993, according to Mother Jones, and the Clinton Health Access Initiative provides vaccines in Kenya, Ethiopia, and Malawi that prevent nearly 50,000 childhood deaths each year. True, Clinton did say in 2008 that "We don't know what, if any, kind of link there is between vaccines and autism," according to that same Mother Jones article, but that's technically true. Wakefield's paper had yet to be formally retracted at that point. Now that it has, she's fully behind vaccines.
Wakefield's paper came under suspicion in 2004, and was finally retracted in 2010. Lancet editor Richard Horton told the Guardian that "the statements in the paper were utterly false," and Wakefield was stripped of his medical license. A year later, it was discovered that Wakefield had devised the hoax in order to drum up sales for a test for a made-up condition, "autistic enterocolitis," which he planned to sell to the tune of $43 million per year, according to CNN. This wasn't a tragic mistake, but a deliberate deception to make money off of unsuspecting parents concerned about their children's health. But still, some people believe Wakefield to this day, including, it seems, the other presidential candidates.
Republican candidate Donald Trump tweeted in 2014 that "many" healthy children develop autism after being vaccinated, and in last year's CNN Republican debate, he even went so far as to claim that he knew a 2-year-old who was vaccinated and developed autism a week later, although he's never provided any proof, of course. The Green Party candidate, Dr. Jill Stein (yes, an actual medical doctor) recently came under fire when she implied during a Reddit AMA that vaccines can't be trusted because the FDA is packed with corporate shills (it is not). She then doubled down in a Washington Post interview, claiming that although vaccines prevent deadly diseases and all, she thinks that "corporate influence and the pharmaceutical industry has a lot of influence" on both the FDA and the CDC. As for Libertarian Gary Johnson, he hasn't mentioned vaccines much since his 2011 "No to mandatory vaccines" tweet, but he's a Libertarian who pledges to "get the government out of your life," so it's safe to say his opinion hasn't changed. For voters who believe in science and keeping kids safe, there's only one choice.