In Europe are vampires and werewolves folklore or an underlying disease? and why?
The folklore is most likely based on diseases or other mental or physical conditions that weren't understood by the people. For example if a body is buried in soil that tends to be a preservative rather than promoting rapid decomposition the people would have arrived at only one conclusion. Then there are people who tend to be extremely light sensitive Then there are people who have rapid abnormal hair growth. As for mental conditions there are people who identify themselves more with animals than they do with human beings. All of that being said there are historical figures such as Vlad the Impaler, Elizabeth Bathory whose actions and behaviours added to the folklore
The stories are based on Rabies, the madness that is caused the disease led to people being banished to the forest and the light sensitivity meant they would howl in pain during daylight hours and could only come out at night. Rabies is contracted by animal bites, predominantly bat and dog bites leading to the superstition that a bite from a bat would turn you into a vampire and a dog/wolf into a werewolf.
Myths and fiction created by writers.
I don't know about vampires but the "werewolf" legend could have a basis in fact because of the disease hypertrichosis. It's an affliction the causes abnormal hair growth all over the body.
They don't exist. They never did. Don't be gullible.
They aren't real - surely you didn't think they were? They're fictional. Pretend. They exist only in stories.