isease, any harmful deviation from the normal structural or functional state of an organism, generally associated with certain signs and symptoms and differing in nature from physical injury. A diseased organism commonly exhibits signs or symptoms indicative of its abnormal state. Thus, the normal condition of an organism must be understood in order to recognize the hallmarks of disease. Nevertheless, a sharp demarcation between disease and health is not always apparent.

The study of disease is called pathology. It involves the determination of the cause (etiology) of the disease, the understanding of the mechanisms of its development (pathogenesis), the structural changes associated with the disease process (morphological changes), and the functional consequences of those changes. Correctly identifying the cause of a disease is necessary to identifying the proper course of treatment. QUIZMedical Terms and Pioneers QuizWho discovered the major blood groups? What causes the blood disease thalassemia? Test what you know about medical science by taking this quiz.

Humans, other animals, and plants are all susceptible to diseases of some sort. However, that which disrupts the normal functioning of one type of organism may have no effect on the other types.

Major distinctions

The normal state of an organism represents a condition of delicate physiological balance, or homeostasis, in terms of chemical, physical, and functional processes, maintained by a complex of mechanisms that are not fully understood. In a fundamental sense, therefore, disease represents the consequences of a breakdown of the homeostatic control mechanisms. In some instances the affected mechanisms are clearly indicated, but in most cases a complex of mechanisms is disturbed, initially or sequentially, and precise definition of the pathogenesis of the ensuing disease is elusive. Death in humans and other mammals, for example, often results directly from heart or lung failure, but the preceding sequence of events may be highly complex, involving disturbances of other organ systems and derangement of other control mechanisms.

The initial cause of the diseased state may lie within the individual organism itself, and the disease is then said to be idiopathic, innate, primary, or “essential.” It may result from a course of medical treatment, either as an unavoidable side effect or because the treatment itself was ill-advised; in either case the disease is classed as iatrogenic. Finally, the disease may be caused by some agent external to the organism, such as a chemical that is a toxic agent. In this case the disease is noncommunicable; that is, it affects only the individual organism exposed to it. The external agent may be itself a living organism capable of multiplying within the host and subsequently infecting other organisms; in this case the disease is said to be communicable.

Noncommunicable disease

Noncommunicable diseases generally are long-lasting and progress slowly, and thus they are sometimes also referred to as chronic diseases. They can arise from environmental exposures or from genetically determined abnormalities, which may be evident at birth or which may become apparent later in life. The World Health Organization (WHO) has identified four major types of noncommunicable disease: cancercardiovascular disease (e.g., heart attackstroke), chronic respiratory disease (e.g., asthma), and diabetes mellitus. WHO estimates that, combined, these four groups of conditions account for 82 percent of all deaths from noncommunicable disease.

Noncommunicable diseases that arise from inherited genetic abnormalities often leave an individual ill-equipped to survive without some form of treatment. Examples of inherited disease include cystic fibrosisDown syndrome, and inborn errors of metabolism, which are present at birth. Examples of inherited diseases that emerge in adulthood include Huntington disease and certain forms of cancer (e.g., familial breast cancer involving inherited mutations in either of the genes BRCA1 or BRCA2).

Metabolic defects

Metabolic defects offer useful insight into understanding the impact of noncommunicable disease on the function of the human body. In humans, for example, the lack of an enzyme known as phenylalanine hydroxylase, which is necessary for the metabolism of the common amino acid phenylalanine, leads to the disease phenylketonuria (PKU), which appears at a few weeks of age and, if not treated, is often associated with the onset of intellectual disability. Other metabolic defects may make their appearance only relatively late in life. Examples of this situation are the diseases gout and type 2 (late-onset, or adult-type) diabetes. Gout results from an accumulation within the tissues of uric acid, an end product of nucleic acid metabolism. Late-onset diabetes results from an impaired release of insulin by the pancreas and a reduction in responsiveness of body tissues to insulin that lead to the inability to metabolize sugars and fats properly.

phenylketonuriaThe blood of a two-week-old infant is collected for a phenylketonuria screening.Eric T. Sheler/U.S. Air Force photo (Photo ID: 071212-F-6244S-292)

Alternatively, the metabolic fault may be associated with aging and the concomitant deterioration of control mechanisms, as in the loss of calcium from bone in the condition known as osteoporosis. That these late-developing metabolic diseases also have a genetic basis—that is, that there is an inherited tendency for the development of the metabolic faults involved—seems to be definitely the case in some instances but remains either incompletely understood or uncertain in others.

Environmental hazards

Metabolic abnormalities also may result from the effects of external environmental factors, a relationship that has been suggested by the apparent confinement of certain diseases to sharply delimited geographic areas. Notable examples are goitre and mottled enamel of the teeth in humans. The development of goitre is attributable to iodine deficiency in the diet, which leads to compensatory growth of the thyroid gland in a vain effort to overcome the deficiency. The disease tends to occur in inland areas where seafood consumption is minimal and dietary supplementation of iodine—through such items as table salt—does not occur. Mottled enamel of teeth results from consumption of excessive amounts of fluoride, usually in water supplies. Conversely, dental caries (tooth decay) is found to occur to a greater extent in areas in which water supplies are deficient in fluoride.Sponsored by Mutual Funds Sahi HaiInvest in verified and well-regulated Mutual Funds!Don’t invest in what is unverified. Invest smartly, in verified and well-regulated Mutual Funds. #VerifiedMutualFunds #RegulatedMutualFundsLEARN MORE

goitreWoman with a goitre.Drahreg01

Analogous conditions in herbivorous domesticated animals result from deficiencies in trace elements, such as zinc and selenium, in the soil of pastures and, therefore, also in plants making up the diet. Similarly, plant growth suffers from soil deficiencies of essential elements, particularly nitrogenpotassium, and phosphorus. These conditions can be corrected by adding salts to the diets of domesticated animals and by applying fertilizers to soil.

There also are diseases resulting from toxic substances added to the environment in sufficient amounts to produce symptoms of greater or lesser severity. Although human disorders of this nature are best known, untoward effects of such contamination of the environment occur also in plants and animals. The problems caused by environmental toxic agents are largely, if not entirely, anthropogenicOccupational diseases, for example, are associated with factors that are present in the work environment or are otherwise encountered in the course of work. Examples of occupational diseases include asbestosissilicosis, and byssinosis, which affect the respiratory tract and which are caused by inhalation of, respectively, asbestossilica, and cotton dust. Also important in this regard are metal poisoning, particularly involving mercurylead, or arsenic; poisoning with solvents used in industrial processes; and exposure to ionizing radiation. Of greater importance to the population at large are the diseases that result from exposure to insecticides and atmospheric pollutants. Such diseases usually, though not invariably, are of a chronic nature; they require prolonged exposure to the noxious agent and develop slowly. Environmental diseases of all kinds, however, also may predispose the individual to other diseases; for example, respiratory diseases such as silicosis render the sufferer more susceptible to tuberculosis.

pneumoconiosisA micrograph of asbestosis showing ferruginous bodies (rust-coloured rods). Asbestosis is a type of pneumoconiosis.Nephron

Communicable disease

Communicable, or contagious, diseases are those transmitted from one organism to another. Infectious diseases are diseases caused in the host by infection with living, and therefore replicating, microorganisms, such as animal parasites, bacteriafungi, or viruses. Practically, these two classes of disease are the same, because infectious diseases generally are communicable, or transmissible, from one host to another, and the causative agent, therefore, is disseminated, directly or indirectly, through the host population. Such spread is an ecological phenomenon, the host serving as the environment in which the parasite lives; complexity arises when the parasite occurs in more than one host species.Sponsored by Mutual Funds Sahi HaiInvest in verified and well-regulated Mutual Funds!on’t invest in what is unverified. Invest smartly, in verified and well-regulated Mutual Funds. #VerifiedMutualFunds #RegulatedMutualFundsLEARN MORE

Infectious diseases are a leading cause of death in children and young adults worldwide. Lower respiratory infections, diarrheal diseases, and tuberculosis are among the most common and deadliest types of infectious diseases. Since the latter part of the 20th century, however, death rates from infectious disease have dropped considerably, because of increases in vaccination and other methods of disease prevention and because of improvements in treatment and in the control of disease spread. Notable progress included the near elimination of polio from the world by the early 2000s and substantial declines in AIDS-related deaths by 2015. As a result, by the second decade of the 21st century, many infectious diseases, particularly those of childhood, were being replaced by noncommunicable diseases of adulthood.

polio vaccine
polio vaccineA health care worker giving a polio vaccine to a child in Katsina state, Nigeria, 2014.Alford Williams/Centers for Disease Control and Prevention (CDC)

Host-parasite relationships

In the context of communicable disease, the host-parasite relationship must be considered not only with respect to the individual host-parasite interaction but also in terms of the interrelationship between the host and parasite populations, as well as those of any other host species involved. Most pathogenic bacteria are obligate parasites; that is, they are found only in association with their hosts. Some, such as staphylococci and streptococci, can proliferate outside the body of the host in nutritive materials infected from host sources. Within the tissues of the host, these organisms set up local infections that spread throughout the body. Still other bacteria, such as the glanders bacillus (Burkholderia mallei) and the gonococcimeningococci, and pneumococci, are more closely adapted parasites, capable of multiplying outside the body of the host only under the artificial conditions of the laboratory. All these microorganisms have complete cell structures and metabolic capabilities.

malaria life cycle
malaria life cycleLife cycle of a malaria parasite.Encyclopædia Britannica, Inc.

A greater degree of dependence on the host is shown by rickettsiae and viruses. Rickettsiae are microorganisms that have the cell structure of bacteria. They exhibit a small degree of metabolic activity outside cells, but they cannot grow in the absence of host tissue. The ultimate in parasitism, however, is that of the viruses, which have no conventional cell structure and consist only of a nucleic acid (either DNA or RNA) wrapped in a protective protein coat. Viruses are obligatory intracellular parasites, capable of multiplying only within the cells of the host, and they have no independent metabolic activity of their own. The genetic information that directs the synthesis of virus materials and certain enzymes enters the host cell, parasitizes its chemical processes, and directs them toward the synthesis of new virus elements.Sponsored by Mutual Funds Sahi HaiStay focussed. Stay invested.Don’t get swayed by market volatility. Stay focussed and invested to reach your financial goals! #MarketVolatility #StayInvestedLEARN MORE

epidemic typhus; Rickettsia prowazekii
epidemic typhus; Rickettsia prowazekiiColourized scanning electron micrograph of the bacterium Rickettsia prowazekii, which causes epidemic typhus and typically is transmitted to humans by certain arthropods, particularly the body louse.AP Images


These various degrees of parasitism suggest that the host-parasite relationship is subject to continuing evolutionary change. The adaptation of the microorganism to its parasitic existence, in this view, is accompanied by progressive loss in metabolic capability, with eventual complete physiological dependence of the parasite on the host.

Parasite specificity

The condition of obligate parasitism is associated with a degree of specificity of the parasite with regard to the host; i.e., the parasite generally is more closely adapted to one species of host than to all others. Microorganisms adapted to plant hosts, with only rare exception, are unable to infect animal hosts, and conversely microorganism parasites of animals rarely occur in plants. A number of host species may be susceptible to infection with a given parasite, and the pattern of host susceptibility need not correspond with taxonomic relationships, including hosts varying as widely as vertebrates and invertebrates.Sponsored by Mutual Funds Sahi HaiStay focussed. Stay invested.Don’t get swayed by market volatility. Stay focussed and invested to reach your financial goals! #MarketVolatility #StayInvestedLEARN MORE

The ability to produce consistently fatal disease in a host is often of negative survival value to the parasite, because it is quite likely to eliminate quickly all available hosts. Consistent with this, there is a tendency for disease resulting from infection to be less severe when adaptation of the parasite to the host has become close. A change in severity of a disease, presumably resulting from adaptation, has been observed in the case of the spirochete that causes syphilis, with the disease in humans being less severe today than it was in the 16th century. However, ecological studies of parasitism indicate that it is incorrect to assume that all host-parasite relationships will evolve toward reduced antagonism and that a resultant disease state eventually will be ameliorated. (For further information see community ecology.)

Disease produced in related host species may be either milder or more severe than in the definitive host. In certain cases, adaptation is so close that the parasite is unable to infect any other hosts under natural conditions; this is true of many microorganisms producing disease in humans. On the other hand, natural infection of secondary hosts may occur, leading to severe or fatal disease. Rabies, for example, is a fatal disease in almost all animal hosts. In some species, such as the bat, however, the virus may persist for long periods as an asymptomatic infection.

Host resistance

The specificity of pathogenic microorganisms with regard to their hosts is an expression not only of differences in microbial character but also of differing host resistance. The ability of a microorganism to produce disease can be evaluated only in terms of the host reaction, and, conversely, the resistance, or immunity, of the host can be judged only with regard to its effect on the microorganism. In short, the two are but different facets of the same phenomenon, and either may be evaluated by holding the other constant and varying it. Commonly, for example, virulence of an infective agent is determined experimentally by inoculating groups of hosts with graded doses of the agent and determining, by interpolation, the dose that produces a typical reaction in 50 percent of the host individuals inoculated. This dose is called the median effective dose, or ED50. It is related in inverse fashion to virulence and in a direct way to resistance. In other words, in a given host, the higher the ED50, the less virulent the infective organism; or, with a microorganism of known virulence, the higher the ED50 with the host it is tested against, the greater the resistance of that particular host. Customarily, in different host species, resistance is expressed as an n-fold increase or decrease (with n equal to a whole number) in the ED50 over that of the normal host species.

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