There are three types of lice that affect humans: head lice, skin lice and pubic lice. However, as the name suggests, only head lice affect the scalp, according to Healthline. It should be noted that head lice may also be found in eyebrows and eyelashes.
According to the CDC, the head louse, or Pediculus humanus capitis, is a blood-sucking, parasitic insect that undergoes three life-cycle stages. It begins as an egg, hatches into a nymph, and finishes life as an adult. The egg, or “nit,” is the smallest, and appears as an oval-shaped knot at the base of the hair shaft, states Healthline. These nits can range in color from white, to yellow, to the color of the person’s hair. Nits are very difficult to see, and are often misidentified as dandruff.
An immature louse, or nymph, looks like an adult but is significantly smaller, while the adult is about the size of a sesame seed. Fully grown lice range from tan, to grayish-white, have six legs, and can live for up to 30 days on a person’s scalp.
As of 2015, it is estimated that 6 million to 12 million infestations occur in the United States each year among children 3 to 11 years of age, according to the CDC.