Tapeworms and Other Parasites Can Make Good Guests
According to the hygiene hypothesis, our ancestors came to tolerate low levels of infection. They even came to depend on parasites to help the immune system develop properly.
.. That’s not to say that parasites became an unalloyed good. Our relationship with them is instead a complicated trade-off.
“Having worms can mean you have fewer allergies when you grow up, but it could also stunt your growth,” said Marlene Zuk, an evolutionary biologist at the University of Minnesota. “Nobody promised you this was going to work out really well.”