Roundworms are parasites that need to live in the body. These parasitic infections can cause diarrhea and fever. Types of roundworms in humans include pinworms and ascariasis. Roundworms are small organisms that can live in your intestine, which is part of your digestive system. Roundworms can live in the human intestine for a long time. They can be harmful and cause many problems, including abdominal (belly) pain, fever and diarrhea.