The wolf at the door
Then there's the “old neighbors” theory. Parasites and hosts have lived together so long, they're used to each other.
You see this with intestinal worms. In developed areas where people have gotten rid of their worms, you have more inflammatory bowel disease. Some people are healthier with worms than without them.
Unless absolutely necessary, I wouldn't want to be infected with worms.
Most people feel that way. It takes a strong gut reaction to overcome the initial gut reaction.