How do pinworms spread through reinfection from eggs under fingernails?

Pinworms spread through a process known as autoinfection or reinfection, which is facilitated by the parasite's life cycle.

When a female pinworm migrates to the anus to deposit eggs on the surrounding skin, the movement of the worm causes intense itching. When an individual scratches the affected area, microscopic eggs are easily transferred and trapped beneath the fingernails.21

Reinfection occurs when these eggs are subsequently passed to the mouth, often through hand-to-mouth contact. Once ingested, the eggs or larvae travel through the gastrointestinal tract to the intestine, where they mature, completing the cycle. Because this transmission follows a fecal-oral route, it is highly contagious and can frequently spread among family members.21