According to the National Center for Biotechnology, tapeworms are some of the largest parasites of man, with the longest growing up to 25 meters (82 feet.) However, most tapeworms grow to a maximum length of 2 to 15 meters (6 to 49 feet).
About 20 million people are infected with tapeworms, also known as cestoda. The parasite spreads to humans when infected food is consumed and is most prevalent in countries where raw and marinated fish is consumed. Humans can also get infected when they eat undercooked or raw pork or beef, according to Centers for Disease Control and Prevention. Tapeworms can live in humans for 20 years, with one documented case of an infection lasting more than 25 years.