Parasitic infections are caused by protozoa and helminth worms. They enter the body through ingestion, arthropod bites, or skin/mucous membrane penetration. Common human parasites include Plasmodium (malaria), Entamoeba histolytica (amoebiasis), Giardia lamblia (giardiasis), and various helminths such as tapeworms and roundworms. Symptoms vary depending on the infecting parasite but may include diarrhea, abdominal pain, fever, and organ damage. Treatment involves antiprotozoal or anthelmintic medications.