An enormous sort of organisms exist, including some which may survive and even develop within the body of individuals or animals. If the organism can cause infection, it's an infective agent . Infectious agents which cause contamination and sickness are called pathogens. Diseases produced by pathogens, or the toxins they produce, are infectious diseases. With the progress of civilization from the ancient to modern man, the concept, of diseases also evolved by stages from super-natural and deistic origin to the natural and 'multifactorial' causation. However we can define disease as "a condition of the body or; some part or organ of the body in which its functions are disturbed or deranged”. It is a discomfort, a condition in which bodily health is seriously attacked, deranged or impaired; a departure from the State of health, an alternation of human body interrupting the performance of vital function. These definitions are considered inadequate because none of them give a criterion by which to decide when the disease state begins and ends. Disease is not a static entity; it is a process with a dramatic or insidious onset, a short or prolonged course and ending in recovery, death or disability. From an ecological point of view, disease is taken into account "mal adjustment of the human Organism to the environment". The disease process is initiated by a disturbance of the balance between man and environment. Illness is also a social phenomenon. It occurs in all societies and is defined and thought: in terms of the particular cultural forces prevalent in a society. The values and customs of a community or social group strongly influence their perception of the symptoms of disease, their interpretation of the symptoms and their techniques for treatment.
