Generally, lice are not considered a health danger in the United States.
Head lice and Pubic (“crab”) lice are not known to transmit any disease.
Although no cases are reported in the United States, body lice have spread epidemic typhus, trench fever, and louse-borne relapsing fever in other parts of the world, notably where civil unrest and/or chronic poverty prevent regular laundering of clothing.
If you scratch too hard, lice bites can become infected. If infected, your skin will be red, tender, and possibly oozing or crusted over. Call your doctor as soon as possible if any of these conditions exist.