Genus Phthirus refers to a group of parasitic insects that feed on human blood. These insects are commonly known as lice, and there are three different species within the genus Phthirus: the head louse (Pediculus humanus capitis), the body louse (Pediculus humanus corporis), and the pubic louse (Pthirus pubis). Synonyms for the genus Phthirus include head and body lice, crab lice, and pubic lice. While these insects are generally viewed as a nuisance, they can also transmit diseases and cause intense itching and irritation. Proper hygiene and use of lice treatments can help to prevent and treat infestations.