Parasites are the group of organisms which reside within or on the body of organisms (hosts) and feed on the host to obtain nutrition and other resources from it. Such type of relationship is referred to as parasitism. Parasitism is somewhat similar to the predation. In that, parasites feed on the host nutrients and causes harm to the host. Such host may become weak and disease prone and ultimately can die. This is a time consuming factor and lasts for few days to few years. But, since parasitism can limit the size of a given population, it is often referred as density-dependent limiting factor. It acts to keep a check on the size of the population.