Humans can also be infested with lice, but the species that affects humans does not affect other animals.
Researchers believe that if the immune system is not properly primed in childhood, parasites like lice can trigger the immune system in humans, making their bodies less likely to react to harmless triggers like pollens and dander.
The study supports the theory that the rise in asthma and allergies can be linked to hyperclean living.
Reuters contributed to this report.
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