The scariest fact I've heard is that there are parasites that can control the minds of their hosts. For example, the hairworm can make its cricket host jump into water, which is suicidal for the cricket but necessary for the hairworm to reproduce. A scary story? There was a story about a doll that was possessed. It was found in an old attic. The family who took it in started experiencing strange things. Their pets would growl at the doll, and at night, they could hear it whispering things they couldn't understand.
Scariest fact: There are certain bacteria that can survive in extreme conditions and are resistant to almost all antibiotics. They could potentially cause a global health crisis. As for a story, there was a ship that was thought to be cursed. Every crew member who had ever sailed on it had either died or gone mad. A new crew was assembled, thinking the stories were just superstition. But soon, they started seeing strange apparitions on the deck and hearing voices. They abandoned the ship as soon as they could, but some of them were never the same again.
A really scary fact is that there are some caves that are so deep and dangerous that people who enter them might never come out. The pressure, the lack of oxygen, and the possibility of getting lost are terrifying. One of the scariest stories was about a bridge. Legend said that if you stopped your car on the bridge at midnight, a ghostly figure would appear in your rear - view mirror. A young couple once tried it out of curiosity, and they both saw a pale face staring at them. They quickly drove away in fear.