Head Lice do not create swarms as do other kinds of insects such as locusts and bees.  Head Lice can, however, live on a human scalp for weeks, and even months, without detection.  All parasites are clever at evading detection, and head lice are no exception.  Currently most people are unaware of effective head lice removal treatment options. This is a huge benefit to the head lice, who continue to thrive on the heads of elementary school children in North America to the tune of 5%-10% at any given time during the year.

PBS Gathering Swarms