I think there's nothing wrong with being disgusted by parasites, it's just an instinct that evolved to stop little guys from taking all your nutrients.
And infectious diseases spread really easily by contact with blood, so the snack packs of the forest are also like public transportation systems for blood-borne pathogens.
But outside of your very understandable desire as an organism to keep your blood inside and pathogens outside, parasites are an amazing and thought provoking aspect of life's diversity.
Wild animals can have dozens of different species of worms and arthropods living on or in them. (Most big animal species even have their own parasites that can only live on that animal.) To the parasite, animals are worlds; a deer is like a planet inhabited by its own fauna, just like deer inhabit the landscape.
Isn't it awe-inspiring that you can go into a habitat and see animals inhabiting it, but each animal IS a habitat with its own animals...
And it keeps going! Parasites often have their own parasites, called hyper-parasites! And hyper-parasites can have hyper-parasites! How many layers of animal are there?!?
Parasites are symbiotic creatures that decrease the fitness of their partner, but "parasite=harmful" is not quite right, since parasitism interlocks the fates of organisms in complex ways. Sometimes a parasite has to spend the first part of its life cycle in one animal, and the second part in another totally different animal. How do they get there? Maybe the second animal eats the first one. The parasite needs this predator-prey interaction to happen to continue life! If the predator turns to other prey, the parasite can't live. But if the predator loses its other prey and has nothing to eat except that prey, well that might seem like a good thing except now there are 27 of you in the same predator's digestive tract, and the predator is now weak and struggles to hunt. If your host starves, you are all dead for certain!
Parasites in a situation with two hosts, one predator the other prey, sometimes might change the prey's behavior to make it easier for the predator to catch. This might be considered helping the predator. Is the parasite "harming" the predator or just taking a cut of the profits when it makes a kill? It's complicated!
Another way to do it is to be a parasite that lives inside a parasite that lives on the outside of an animal, and when the animal grooms itself and bites the ectoparasite out of its fur, the parasite living inside the parasite can now grow inside the host animal.
Parasites' impact on their hosts' behaviors impacts the whole environment! For example maybe a herd of deer likes to browse on the tender shoots down in the swamp, but they do not like the swarms of mosquitoes. By driving off the deer, the mosquitoes help the orchids in the swamp survive. If a bison wallows in the dirt to get rid of parasites, it creates a disturbance that gives rise to a little mini habitat for flowers that can't survive in the tall, less disturbed grass. If parasites make it unhealthy for animals to live crammed in a small area, they might be driven to disperse and move to new habitats, or to have a system of migrating from place to place. If a large animal is itchy and scratches itself against a tree and rubs the bark off, that might kill the tree, which is bad for the tree but great for the woodpeckers that need standing deadwood to hunt for food.
Speaking of woodpeckers, we have recently learned that woodpeckers transmit lichens and mosses to new places! And woodpeckers also were found to harbor freshwater diatoms...which should be found in freshwater streams, but perhaps got onto the bird when it was taking a bath...why does a bird take a bath? Perhaps to get rid of parasites...?
...Basically, everything is so interconnected that a flea could affect an unimaginable number of things. Parasites weave together the ecosystem in ways we barely understand.
Of course, you should still treat your dog or cat for fleas...but that's part of your symbiosis with the dog or cat, so even the space where a flea should be, is a space where organisms are bound together.
