Head lice spread when a live louse (most likely an adult) makes its way onto your head. But when a louse starts laying eggs (also called nits) on your hair, that's when an infestation really takes off.
A female adult louse will lay about 4 eggs per day. When she lays the egg, she will secrete a quick-setting glue that firmly attaches the egg to a single hair shaft, within about a quarter inch (~0.5 cm) of the scalp. The egg stays there until its lid (called the operculum) pops open 8-9 days later and a louse nymph crawls out to feed on human blood from the scalp below.
Lice eggs are very small, hard to see (see lice egg pictures) and are well protected. Because of the glue, they are difficult to manually remove with anything but a lice comb (and even then it is hard for a novice to get them all). And their shells protect them well from lice pesticides, herbs and suffocation products, making them very hard to kill.
The reason why most lice-product treatments require follow-up applications a week or so later is because the lice eggs resisted the initial treatment and the lice need to be killed after they hatch from the eggs.
One effective lice nit treatment is the professional AirAllé™ lice device, which dehydrates the eggs through heated air. Because the AirAllé device kills 99.2 percent of lice eggs in a single treatment, the likelihood of needing a follow-up treatment is statistically very low. Learn more about how to get rid of lice eggs.