There are all sorts of parasites. some are animals, some plants, some fungi and many bacteria and viruses. Not all of them suck blood, and not all of them are that harmful. Some do so little harm they are usually called commensals instead of parasites. For example, we have tiny mites living inside the base of our eyelashes etc. They don't do any harm at all. But a very similar mite on dogs causes the terrible disease of mange.
Many parasites look gross, at least to us; but I supppose it's the thought of an "alien" living inside us and drinking our blood or eating us alive that's the most horrible thing about them. Especially if you watch too many horror films!
I find the thought of meeting a "camel spider" rather scary. They aren't poisonous and would not be able to do a human much harm, but they have four huge "jaws" called chelicerae that move independantly to munch on their prey (or give a painful bite to a person). They also move extremely fast compared to most creatures of their size.
I would probably be scared of the Scolopendra centipede too. It is poisonous (though bites aren't usually fatal to humans) as well as fast moving. The biggest can grow to a foot long or more, and I would definitely not want to go where they are likely to be found! http://en.wikipedia.org/wiki/Scolopendra_gigantea