Which of the following best describes lice?

Pediculus humanus (head or body louse) and Phthirus pubis (crab louse) are wingless parasites exclusively affecting humans. Lice are important not only for the itching and discomfort they cause but also for the diseases they transmit. These disorders include epidemic typhus, relapsing fever, and trench fever. There is no evidence that lice secrete toxins.