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Parasites refer to organisms that live on or in another organism, known as the host, and derives its sustenance at the host's expense. They can be found in various forms and sizes, ranging from single-celled protozoans to complex multicellular animals such as tapeworms and fleas. Parasites can cause harm to their hosts, either by direct damage to tissues or by transmitting diseases. Parasitism is a type of symbiotic relationship where one organism benefits at the expense of another. Parasites can be classified based on their location on or in the host, such as ectoparasites (living on the host's surface) and endoparasites (living inside the host). They can also be classified based on their mode of transmission, such as direct life cycle parasites (transmitted directly from host to host) and indirect life cycle parasites (requiring an intermediate host or vector for transmission). Parasites have evolved complex adaptations to exploit their hosts, including specialized structures for attachment, secretion of toxic substances to evade the host's immune system, and manipulation of the host's behavior. They can also have significant impacts on ecosystems, affecting host population dynamics, community structure, and nutrient cycling. Studying parasites is important for understanding their ecology, evolution, and impact on human health. Parasitologists use various techniques to identify, classify, and analyze parasites, including microscopy, genetics, and epidemiological surveys. Prevention and control of parasitic diseases often involve a combination of medical interventions, such as drugs or vaccines, and public health measures, such as improved sanitation and hygiene practices.