Disease is a condition that disrupts the fine-grained physiological balance that exists among chemical, physical and functional processes within the organism. This imbalance can lead to a number of symptoms and signs, as well as a host of health issues in the individual and those who come into contact with them. The study of Disease is known as pathology, and it involves the determination of the cause (etiology) of the disease, the mechanisms of its development (pathogenesis), and the structural changes that occur with the process of disease. Disease can affect all types of living things, including humans, animals and plants.
The process of Disease can be described as a “horse race” between the infectious microbe, or pathogen, and the host’s immune system. The pathogen attempts to gain a foothold in the organism by entering the skin or internal mucosal surfaces of the respiratory, digestive and urinary tracts. Once a foothold is established, the pathogen begins to reproduce and spread, while the immune system attempts to eradicate or contain it. The length of time between the entry of the pathogen and the appearance of symptoms is called the incubation period.
The definition of Disease may also include a number of other aspects, such as the cause of the disease, the duration of the disease, and the way in which it is transmitted. Some diseases are acquired over a lifetime, such as psoriasis or diabetes mellitus type 1; others are present at birth, and therefore are called congenital. Other diseases are caused by lifestyle choices, such as poor nutrition or a sedentary life; these are known as lifestyle diseases. A final classification is the idiopathic disease, where the exact cause of the disease is unknown.
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