From the Latin verb scabō, meaning "I scratch, scrape," a scab is a crust of hardened blood and serum that naturally forms over a wound. Scab was a natural word to describe the feeling of the itchy protective scale once it forms over a cut or scrape. This same root also gives us the word scabies, the name for the contagious itch (especially with scaly crusts) caused by parasitic mites.