protozoa
STIs may be caused by several different types of pathogens, including protozoa, insects, bacteria, and viruses. For example: Protozoa cause an STI called trichomoniasis. The pathogen infects the vagina in females and the urethra in males, causing symptoms such as burning and itching. Trichomoniasis is common in young people. Pubic lice, like the one in Figure below , are insect parasites that are transmitted sexually. They suck the blood of their host and irritate the skin in the pubic area.