There are a large number of tapeworms that are known to affect humans. All tapeworms initially attach themselves to the intestinal walls of humans, from where they access nutrients as the food eaten by the victim is digested, and grow in size.
Some varieties of tapeworms are relatively harmless as they only reside in the intestines and are in due course expelled from the body of the victim with fecal matter. Though if the worms reside in the intestine for a long duration of time, they grow large in size and can block the bile duct.
The more dangerous varieties of tapeworms can migrate to other organs and tissues like the muscles, lungs, liver, eye and even the brain. Here they lead to the formation of cysts which gradually grow larger in size and eventually lead to organ failure and can even cause death.
Tapeworms are not beneficial to humans in any way.