A parasite is an organism that lives on or inside another organism, known as the host, and benefits at the host's expense. Parasites can be found in various forms, including protozoa, worms, and ectoparasites like fleas and ticks. They often rely on their hosts for nutrients and can cause harm, leading to diseases or weakened health. Parasites have complex life cycles and can reproduce quickly, making them difficult to control. Some common examples include malaria caused by the protozoan Plasmodium, and tapeworms that inhabit the intestines of animals and humans. Understanding parasites is crucial for health and disease prevention.