Parasitism describes a biological interaction where one organism, the parasite, benefits at the expense of another organism, the host. This relationship is characterized by the parasite acquiring necessary resources, such as nutrients, shelter, or reproductive capacity, without providing a corresponding benefit to the host. Parasites can range greatly in their life cycles and modes of transmission, existing as ectoparasites (living on the host's exterior) or endoparasites (living within the host's body). The resulting impact on the host can vary widely, causing symptoms ranging from mild irritation to severe systemic illness, depending on the parasite's reproductive strategies and the intensity of the host's immune response. Pathogenesis, the mechanism by which the host suffers detriment, is determined by the parasite's ability to evade or suppress the host's internal defenses while sustaining a population density sufficient for transmission.