Pediculus humanus capitis, commonly known as head lice, are tiny parasitic insects that infest the human scalp. They feed on blood and can cause itching and discomfort. Head lice are most often spread through direct head-to-head contact, making them common among children in schools and daycare settings. These insects are about the size of a sesame seed and can be difficult to see. They lay eggs, called nits, which attach to hair shafts close to the scalp. While head lice do not transmit diseases, they can lead to secondary infections from scratching and may require treatment with special shampoos or lotions.