Parasites in the Human Body: What Science Actually Says Parasites can live in the human body, but their effects vary widely. Many infections are mild, treatable, or even symptom-free. Here’s a clear, evidence-based look at what some common parasites can do — not what they always do. Tapeworms • Live in the intestines • Can absorb nutrients from food • Heavy infections may lead to vitamin deficiencies, but many people have no symptoms Roundworms (Ascaris) • May cause abdominal pain, bloating, or