A disease is an abnormal condition of a living organism, caused by organic or functional alterations, which compromise its health. In many cases, terms such as disease, disorder, morbidity, sickness and illness are used interchangeably. The concept of disease implies the consideration of the concepts of “normality” and health; the latter, as far as human beings are concerned, has a classical definition found in the Constitution of the World Health Organization.
We can define “disease” as the opposite of health: an alteration of the physiological state of an organism (possibly including the psychological one) capable of reducing or negatively modifying the normal functions, combined with the complex of physiological reactions resulting from the pathological state. In the definition of disease is fundamental the principle of transience: every disease has a term that can be represented by the healing of the organism, by the adaptation of the same to a different physiology (or a different condition of life) or death.
The term derives from that of “sick”, which in turn comes, by crasis and alliteration, from the Latin “male aptus” translatable into “battered – battered”, and from male-actio = bad-action = disease induced by wrong action, due to ignorance of the mind of the subject. Passing then from the etymological meaning to the “real” one of the term, that is to say to its definition, we meet many difficulties, because it is one of those definitions apparently simple and easy, but in reality very difficult to give, especially in the official medicine.
The diseases that can affect humans are numerous and can be classified with various criteria. A first distinction is made between congenital and acquired diseases, indicating with the first term those diseases that are already in place at birth and with the second term those diseases that are contracted after birth.
Some congenital diseases are actually acquired during fetal life, such as some malformations induced in the fetus by infectious diseases contracted by the mother during pregnancy. Congenital in the strict sense of the word, or rather hereditary, are instead those diseases that are already present in a potential state at the time of the formation of the new individual, i.e. they are included in his genetic heritage.
The limits between congenital and acquired diseases are extremely imprecise; in fact, many agents of acquired diseases are able to cause a disease only if the individual is predisposed, that is, if he already has the tendency to accept the pathogen or to react to it in an exaggerated manner compared to the average of the population. Acquired diseases can also be classified on the basis of their etiology: we will thus speak of diseases caused by physical agents (burns, traumatic injuries, diseases of radiation, electricity, etc.. Thus, we will speak of diseases caused by physical agents (burns, traumatic injuries, radiation diseases, electricity, etc.) or chemical agents (poisoning by organic or inorganic substances), by pathogenic schizomycetes that sustain many infectious diseases (typhoid, syphilis, tuberculosis, etc.) and by animal parasites (responsible for malaria, trypanosomiasis, etc.) or plant parasites (responsible for mycosis), by filterable viruses.
Another criterion for the classification of diseases is based on the elementary pathological process that characterizes the morbid condition: thus are defined inflammatory diseases (indicated with the suffix -itis) those sustained by inflammatory processes (pneumonia, appendicitis, etc..); degenerative or regressive diseases (indicated with the suffix -osis) those dominated by degenerative facts of one or more organs or tissues (steatosis of the liver, arteriosclerosis); febrile diseases those that have as a peculiar symptom a strong rise in temperature.
An important group of diseases is constituted by neoplasms characterized by undifferentiated autonomous proliferation of cells. A further classification is possible on the basis of the system or organ that appears particularly affected: there will be diseases of the respiratory, cardiovascular, digestive, etc..; diseases of the metabolism, supported by a deficit of intermediate metabolism; endocrine diseases, affecting the internal secretion glands; diseases of food origin, caused by insufficient nutrition or lack of vitamins.
A very common distinction, finally, is that between organic diseases and functional diseases: the first show obvious morphological alterations of organs, the others only functions more or less disturbed.
A particular group is formed by contagious diseases, i.e., those transmissible by contact with affected individuals or their products of secretion or excretion. Another important group is constituted by occupational diseases, i.e. those contracted due to the harmful action, slow and protracted over time of substances or materials handled and tools used at work (inhalation of dust, vapors or fumes, skin contact with irritating materials, occasional ingestion of toxic substances; from abnormal noise or excessive vibration of plants, etc.). Depending on the case they are characterized by intoxication, irritative phenomena, injury to organs or apparatuses. For occupational diseases see the specific entries: hydrargyrism, silicosis, etc., as well as arsenic, mercury, etc..
Various diseases are marked by the name of the authors who first described them, by the most conspicuous clinical sign, by the place where they were found for the first time or are widespread, by the professional category that is particularly exposed, etc..