Those who have had to argue with anti-vaxxers will know that the argument shifts. If it's not mercury in the vaccines that's causing problems, it's dead fetus parts, and if it's not dead fetus parts, it's unspecified "toxins." But did you know the free-form argument has been going on longer than you imagined? .
It began with smallpox. Smallpox, until its eradication, had a mortality rate of 30%, and those it spared were often left blinded. Everyone had a theory on how it could be prevented. But in the eighteenth century, Edward Jenner noticed that those with cow pox rarely caught smallpox, and decided that the far milder disease was the key to defeating its deadlier variant.
His first test subject was an eight-year-old boy, because it was the cusp of the 19th century, and medical ethics hadn't really become a concern for anyone, yet. Taking some of the contents of a pustule on a dairy maid's hand, Jenner injected the eight-year-old. He then exposed the kid to smallpox several times. The boy never fell sick.
Clearly, there was a lot to object to in Jenner's method, but he had good results. How could people argue with them? By using nonsense — Biblical nonsense. No, they didn't quote scripture at Jenner; they said that injections of cow pox caused boanthropy.
The first recorded case of boanthropy was King Nebuchadnezzar, who expelled the Jews from Jerusalem. He built the Hanging Gardens of Babylon, and, apparently, bragged a bit too much about them, because it was at that point God had had enough. Nebuchadnezzar "was driven from men and did eat grass as oxen." He lived as a cow for seven years, after which he was a little more inclined to worship the right deity.
It wasn't much, but it was enough for Jenner's opponents. That's right — they decided that, if in your arrogance you inject your children with a cow disease, you will turn them into cows. The kids would eat grass, crawl around mooing, and hit people with their horns. Some people even said that the cow pox inoculation would make kids grow horns. (Let me tell you, if they did? I'd get it right now. I'd never lose my keys again and have an available weapon at all times. Plus I think I'd look good with horns.)
There was a lot of skepticism about this vaccination method at the time. Unlike now, though, there was more fear of the effects of the disease than there was fear of the effects of the vaccination. Jenner's method caught on, and became one step in the other difference between the 19th century and now, no more smallpox. Because of vaccinations.