Internal parasites harm their hosts in many ways. Some suck blood and others destroy body cells. Excessive parasitic loads can lead to obstructions in the intestine, bile ducts and blood vessels. Some parasites can produce toxic substances and cause allergic reactions. Heavy parasitic loads can cause stress and suppress the immune system - opening the door to a wide variety of infectious bacteria.
Heavy infections can also affect the fertility level of eggs and thus reproduction.
The worms reproduce in the host, and the eggs are excreted in the feces. Most occur in the intestine and stomach though some larva can migrate into the lungs and throat. Those most commonly found are 'roundworms' which have an indirect life cycle. Hookworms have a direct life cycle and huge loads can build up in the system in a very short time. Pinworms also have a direct life cycle and commonly cause an obstruction in the intestinal canal.