Various definitions of the term "illness" are offered many:

1. K.Marks - " Illness is the life constrained in freedom " .Р.Вирхов - " Illness is a life under abnormal conditions ".

I.V.Davydovsky - " Illness is the adaptation of an organism described by specific forms and levels of adaptive activity ".

A.D.Ado - " Illness is a life of the damaged organism at participation of processes of indemnification of the broken functions. Illness reduces work capacity of the person and is qualitatively new process ".

N.N.Zajko - " Illness is infringement of normal ability to live of an organism at influence on it damaging agents therefore its adaptive opportunities " go down.

From the resulted definitions of concept "illness" follows, that:

- The reasons of illnesses are pathogenic factors of the external or internal environment, and the leading part in occurrence of illness belongs to factors of an environment even if it and concerns hereditary diseases;

- For illness insufficiency of adaptive reactions of an organism is characteristic. They can even be significant strengthened, but are insufficient for maintenance of balance of an organism with an environment;

- Illness is characterized by decrease in work capacity and social utility of the person. Short-term decrease in work capacity owing to overfatigue, and also last months pregnancy should not count reduction of social utility of the person. It is necessary to mean, that downturn of work capacity as the attribute of disease, is not characteristic for illnesses of children and old men.

So, illness is the original vital process arising under influence of nocuous factors working on an organism and expressing in a complex of structurally functional and metabolic changes, and also infringement of functions and adaptability, restriction of serviceability and social - useful activity.

N.I.Losev defines the following of illness: " Illness is the dynamic condition of an organism described by infringements of normal current of vital processes, resulting to decrease in biological and social opportunities of the person ".

Illness differs from pathological reaction, pathological process and a pathological condition.

Pathological reaction - inadequate and biologically inexpedient answer of an organism or its systems to influence usual or extreme stimulus. For example, short-term increase the arterial pressure after a nervous or downturn of presure a level of sugar in blood in connection with introduction of the big dozes of insulin.

Pathological reactions are either consequence, or display of the broken reactance of an organism. In one cases they can have qualitative character as the perverted, paradoxical answers to the certain influence or inadequate on intensity and duration of reactions. For example: allergic reactions both inadequate psychoemotional and behavioural reactions.

The pathological condition is a proof deviation of structure and function of body (fabric) from the norm, having biologically negative value for an organism; the infringements poorly varying in time. Pathological conditions can be genetically determined (defect of a upper lip and the firm sky, etc.) and can be consequence before the transferred disease or pathological process: consequences of traumas - cicatrixes, loss of finiteness, ankylosis, a lameness, false joints; a tuberculosis of a backbone - a hump; a rickets - deformation of a skeleton).

Examples of pathological conditions are also stump (after amputation of finiteness), cicatricial changes of fabrics after a thermal burn, an atrophy of alveolar shoots of a jaw in connection with removal or loss of the teeth, the got defect valval the device of heart.

Under influence of various additional influences the pathological condition can sometimes proceed in quickly developing pathological process.

Pathological process is sequence of reactions naturally arising in an organism on damaging action of the pathogenic factor. The same pathological process can be caused by various etiological factors and be a component of various diseases, keeping thus the essential distinctive features.

Examples of pathological processes are an inflammation pulmonary fabrics at a pneumonia, an inflammation of a cardiac musle at a heart attack of a myocardium, a fever at a belly typhus etc.

Set of pathological processes defines pathogeny diseases. But illness is not the simple sum of pathological processes.

Prominent features of pathological processes:1. Pathological processes can represent itself as an early stage of development of illness (adjournment of cholesterol as display broken lipide exchange, a thrombosis of coronary arteries as display of the broken microcirculation a heart attack of a myocardium).