Jet Bin Fever Pinworms! Pinworms everywhere! Enterobius vermicularis!
Gravid female pinworms will migrate to the host's asshole and literally explode their uterus out releasing eggs and a chemical irritant over the skin, making it incredibly itchy and nearly impossible not to want to scratch. Then the child, not knowing better, will scratch their butts and get the eggs under their fingernails or stuck on their hands... and as kids do they will then eventually put their hands in their mouth, reinfecting themselves. The lifecycle continues!
I didn't study anything in Helminthology about face eating though, so this is a bold new vision for pinworms!