Parasites are organisms that live on or inside a host and derive nutrients at the host’s expense, sometimes causing disease. In humans, parasites are commonly classified into three main groups: protozoa (single-celled organisms such as Giardia), helminths (worms such as roundworms, tapeworms, and flukes), and ectoparasites (external organisms such as lice and mites). Transmission can occur through contaminated food or water, insect bites, contact with infected individuals or animals, or poor sanitation. Symptoms vary widely depending on the type and location of infection and may include gastrointestinal disturbances, fatigue, anemia, skin irritation, or organ-specific complications. The severity of illness depends on the parasite involved, the level of exposure, and the individual’s immune status.