Things that go bump

Horrific as changeling and body-snatcher myths are, they were, in their time, far less scary than just not knowing. I mean, imagine you're a mom in the Philippines in the 1800s - you put a healthy baby to bed, and he never wakes up. You don't know what SIDS is, much less how to prevent it - all you know is that your grinning, perfect baby has been replaced by a body. You don't understand why miscarriage happens. So you come up with a story about a fiend who sucks fetuses out of their mothers and steals babies from their beds. And then you build into the myth some charms and wards so that you don't have to feel totally helpless. 
The changeling myth is actually somewhat kinder. When the fairies stole a child, they generally took good care of it. And how did humans repay this? Set the fairy babies on fire. Gratitude. 
Folklore has always tried to fill the vacuum in humanity's understanding of life and death. 
TB is a horrible disease. It was called consumption because folks had to simply watch the disease consume their loved one over the course of months or even years, until they were too thin to stand and too weak to breathe. Nobody knew what caused it, what caused some to survive longer than others, or how to treat it. To fill the void of understanding, some folks naturally concluded vampires. They do consume people under cover of darkness while everyone's asleep, after all. There used to be these vampire panics where people would start digging up dead people and setting fire to their hearts. When, in 1892, it became clear that Edwin Brown's consumption was going to kill him as it had his mother and two sisters, his neighbors knew just what to do. They dug up his mom and sisters, and based on the decomposition of the bodies, determined that his sister Mercy Brown had been the vampire. So they cut out her heart, burned it, and made Edwin drink a potion laced with the ashes. He died two month later. The kicker - science had known for ten years that a bacillus called  M. tuberculosis caused the disease, but the people in Edwin's Rhode Island town hadn't yet gotten the memo. 

Yeah, old timey people sure were dumb... maybe even slightly dumber than us. I mean, taking shark supplements to treat cancer is way less horrific than drinking the heart of your dead sister to treat consumption, but it's equally pointless. Although Mercy Brown, on account of being dead, actually was now immune to consumption (unlike sharks, which totally do get cancer), so old timey folks do have us there. We don't know exactly why some people get cancers and others don't, so we make up potions of protection. 
We don't know why autism exists, so we invent this bug bear, Big Pharma, that is magically strong enough to silence every scientific institution on the planet, but not strong enough to silence random b-list celebrities. And actually, denying our children vaccines is similar to holding the proverbial changeling over the fire. Except when you don't vaccinate your own kids, you're not just holding your own children over the fire, but every unvaccinated and immuno-compromised child they meet as well. Okay, maybe old timey people don't have the corner market on crazy.

You think you know fear?

(Woah, I just realized something. Old timey people believed that cats could steal babies' breath. And actually, though rare, it is possible for a cat to "snuggle" a newborn to death... effectively stealing its breath.)