Lice

Itching is the most common symptom produced by lice, and is explained by our allergic reaction to the saliva secreted by the lice when they suck our blood (when they bite us).

This is why we spend the day scratching our head when we have lice. And scratching, scratching all day long won't solve the problem. Indeed by scratching we can cause small sores that don't help to lessen the itching sensation.