This relationship between host and associate been hinted at already and can be as the association between organisms whereby only one, the parasite, , the partner suffering some definite harm. There often some loss of free life on the part of the parasite, and the association between host and parasite frequently of long duration when judged against the life span of the organisms involved.
Parasites four features that collectively identify them as such. Firstly, they in or on a host, and it harm. The depth to which they the host varies, as indeed the damage. Fleas, leeches and lice on the surface and cause superficial injury. Athlete's foot is a skin disease by a fungus living in the surface layers of the foot. The parasite of sleeping sickness is in the host's blood wriggling between blood corpuscles. Secondly, parasites some simplification of body structures when compared with free-living relatives. Sacculiria (a relative of the crab) loss of limbs and is to a mass of reproductive tissue within the abdomen of its crustacean host. Dodder, a plant parasite, leaves, roots and chlorophyll. Thirdly, although all organisms adaptations to their way of life, in the case of parasites they are often with a complex physiological response, e.g. the ability to survive in regions almost devoid of available oxygen, such as adult liver flukes, or the hooks and suckers of adult tapeworm. Lastly, parasites a complex and efficient reproduction, usually associated in some way with the physiology of the host, e.g. rabbit fleas are by the level of sex hormone in their host.
Many authorities that the most damaging and traumatic parasitic associations are probably relatively recent relationships - the participants not yet having had time to &"settle down"; to the parasitic way of life. Obviously it is of no value to the parasite to seriously damage, or even kill, its source of food and life.
Parasites a long history of association with man, and such finds as Egyptian mummies have evidence of parasitic infections that were obviously present in people many thousands of years ago. It should be that often the mummies were the remains of wealthy people who presumably a full and healthy life by the standards of that time, and hence one can the state of people living in dirty and impoverished conditions.

(J. M. Hard, Biology, Hamlyn, 1975, pp. 34-35)