Only humans can be infected by adult T. solium and T. saginata tapeworms. Taeniasis is acquired by humans through the inadvertent ingestion of their cysticerci in undercooked pork or beef, respectively. Once in the human body, cysticerci develop into adult worms that live in the intestine and release egg-bearing gravid proglottids (segments) which are passed out with faeces. Cysticercosis is acquired when proglottids or eggs are ingested. It is a natural infection of pigs and cattle, but, in the case of T. solium, it can also affect humans, usually when they swallow T. solium egg-contaminated soil, water or food (mainly vegetables).
Taeniasis and cysticercosis are common in areas where animal husbandry practices are such that pigs and cattle come into contact with human faeces. The frequency of both conditions has decreased in developed countries owing to stricter meat-inspection standards, confinement of livestock, improved hygiene and better sanitary facilities.