Amoebae and Giardia lamblia are spread via human faeces. The pathogens can enter the body if traces of excrement enter the mouth. In southern countries, this often occurs through dirty food or drinking water. The parasites can also be transmitted during sex.
Threadworms live in the large intestine. The females, however, emerge in the night to lay their eggs between the buttocks. If you scratch the itchy areas in your sleep, the eggs can get under your fingernails and from there can again enter the mouth, worsening the infection. The eggs can also be passed on to other people. That can easily happen during sex, especially from licking the vagina or anal area ("rimming"). The eggs can also be transmitted via foodstuffs, bed sheets and towels. Even so-called “dust infections” can occur, when the eggs are dispersed by shaking out the bed sheets.