When a head louse inserts its mouthparts into human skin to obtain blood, it injects saliva that may cause intense irritation and subsequent itching of the scalp. Children less than 12 years old typically are more sensitive to head-lice bites than older children and adults.
Scratching of head-lice bites increases inflammation, sometimes leading to secondary bacterial infections. However, head lice are not known to be responsible for the direct spread of disease organisms.