For eight months, a 35-year-old man in India was bothered by his left eye. It was red and blurry. When he finally visited an ophthalmology clinic, it didn’t take long for doctors to unearth the cause.

Threadworms spend up to 10 minutes exploring human skin before finding the best place to burrow in. When a specific dopamine signal is blocked, the worms almost completely stop trying to penetrate the ...