A flea is a parasite, and it’s first preference for food is the blood of a host. But if it cannot obtain blood, it will eat almost any organic material including clothes in order to survive.
This is especially true for juvenile fleas who emerge from the eggs as flea larva. They are blind and will feed on any organic substance nearby including the feces of adult fleas or the hair of its hosts, or even clothes.
Flea eggs can remain on clothes for months or even years in a dormant state only to emerge later and become active when the environment becomes more conducive to its existence.
If you have recently ridded your home of a flea infestation, you can remove any dormant flea pupa from your clothes by washing them in hot water with detergent and bleach, and then putting them in the dryer on high heat.