Interesting fact. There are more parasites than any other life form. Think about it. Every organism on the planet has at the very LEAST one, and usually many many more lifeforms that parasitize it. There's a little ditty based on some verse by Jonathan Swift -" Big fleas have little fleas upon their backs to bite' em. And little fleas have lesser fleas, and so ad infinitum."
We humans are parasitized by fleas, ticks, face mites, pin worms, and many other relatively harmless creatures. We are also host to some more serious parasites, such as tapeworms, roundworms, filaria worms, and liver flukes, among many many more. For all of these nasty biting blood suckers and thieves, it is their ability to infest us in great numbers that causes problems. We have evolved to tolerate moderate predation by most parasites and a tapeworm or a couple of roundworms, disgusting though they be, won't hurt us too badly. There is even a school of thought that our immune systems need to be challenged to some degree by some sort of worm. According to this hypothesis -sometimes called the hygiene hypothesis- autoimmune diseases such as asthma, allergies, Crohn's disease, and others are caused when the immune system lacks exposure to parasites and bacteria in the environment.
But when the parasite load becomes too great, we are weakened and likely to succumb to predators, or illness.
There are some parasites that are much much more deadly. There is, believe it or not, a parasitic barnacle, called Sacculina. This barnacle creature injects itself into a crab's body and begins to grow roots, or tentacles that invade the crab, feeding from its blood, but keeping the crab alive. It takes over the crabs body systems, and rides the animal, but invisibly from the outside.
Other parasites are smaller, like the toxoplasma gondii that gets picked up by rodents, and then subtly alter the rodent brain in such a way that it becomes likely to be eaten by a cat. The parasite then infects the cat where it finishes out the reproductive cycle.
I hear conservatives complaining all the time about how they don't want to be taxed in order to support single moms, or students, or lazy crack addicts, you name it. Right-wingers call them parasites on society. Maybe so. But they are the face mites on society. The real parasites we need to fear are more subtle and dangerous than that. They are ones who worm their way into the brains, the government, and take over. They use their influence to write laws (or have laws written by their slaves, bought and paid for) that change the mission of the US government. The real parasites don't want our resources to go to promoting the general welfare, but rather to enriching themselves even more. They are the Wall Street execs, the bankers, the oil barons.
We can stand to support a few crack addicts, to provide food stamps to lazy bums, or mentally ill people, or students, or old sick people. They won't bleed us dry. But the bankers and corporate CEOs will kill us if we let them.