Disease

From Wikipedia, the free encyclopedia

"Contagious" redirects here. For the Isley Brothers song, see Contagious (song).

The term 'disease' refers to any abnormal condition of an organism that impairs function. In human beings, "disease" is often used more broadly to refer to any condition that causes discomfort, dysfunction, distress, social problems, and/or death to the person afflicted, or similar problems for those in contact with the person. In this broader sense, it sometimes includes injuries, disabilities, disorders, syndromes, infections, symptoms, deviant behaviors, and atypical variations of structure and function, while in other contexts these may be considered distinguishable categories.

The goal of medicine is to where possible prevent, and otherwise to delay, disease rather than treating people once disease has become established. Doing this has been possible when preventing disease that is caused entirely by a foreign infectious agent (e.g. smallpox) and explains the huge success of vaccines that have eradicated diseases caused by agents such as smallpox (smallpox was declared as eradicated at the end of 1979 [1]).

Contents

[edit] What causes disease?

Causes of disease can be considered as a spectrum where at one extreme disease is caused entirely by genetic factors (e.g. CAG repeats in the Huntingtin gene that can cause Huntington's Disease) and at the other extreme is caused entirely by foreign agents (e.g. Smallpox). In between these extremes genes (e.g. NOD2/CARD15) and foreign agents (e.g. Gut microbiota) interact to cause disease, as seen for example in the inflammatory bowel disease Crohn's Disease (Fig 1, right).

Figure 1. Causes of disease
Figure 1. Causes of disease

Absence of the genetic or environmental factors in this case results in disease not being manifest. Koch's postulates can be used to determine whether a disease is caused by an infectious agent.

To determine whether a disease is caused by genetic factors, researchers study the pattern inheritance of the disease in families. This provides qualitative information about the disease (how it is inherited). A classic example of this method of research is inheritance of hemophilia in the British Royal Family. More recently this research has been used to identify the Apoliprotein E (ApoE) gene as a susceptibility gene for Alzheimer's Disease, though some forms of this gene - ApoE2 - are associated with a lower susceptibility. To determine to what extent a disease is caused by genetic factors (quantitative information), twin studies are used. Monozygotic twins are genetically identical and likely share a similar environment whereas dizygotic twins are genetically similar and likely share a similar environment. Thus by comparing the incidence of disease (termed concordance rate) in monozygotic twins with the incidence of disease in dizygotic twins, the extent to which genes contribute to disease can be determined. Candidate disease genes can be identified using a number of methods. One is to look for mutants of a model organism (e.g. the organisms Mus musculus,Drosophila melanogaster, Caenhorhabditis elegans,Brachydanio rerio and Xenopus tropicalis) that have a similar phenotype to the disease being studied. Another approach is to look for segregation of genes or genetic markers (e.g. single nucleotide polymorphism or expressed sequence tag) (Fig. 2).

Figure 2. Genetic markers help locate a disease gene
Figure 2. Genetic markers help locate a disease gene

A large number of SNPs spaced throughout the genome have been identified recently in a large project called the HapMap project [2] [3]). The usefulness of the HapMap project and SNP typing and their relevance to society was covered in the 27th October 2005 issue of the leading international science journal Nature (Fig 3).

Figure 3. A special issue of the leading science journal 'Nature' focusing on results from the HapMap project
Figure 3. A special issue of the leading science journal 'Nature' focusing on results from the HapMap project

A large number of genes have been identified that contribute to human disease. These are avaialble from the US National Library of Medicine, which has an impressive range of biological science resources available for free online. Amongst these resources is Online Mendelian Inheritance in Man - OMIM that provides a very, very comprehensive list of all known human gene mutations associated with, and likely contributing to, disease. Each article at OMIM is regularly updated to include the latest scientific research. Additionally, each article provides a detailed history of the research on a given disease gene, with links to the research articles. This resource is highly valuable and is used by the world's top science researchers.

[edit] Useful Terms

Medical usage sometimes distinguishes a disease, which has a known specific cause or causes (called its etiology), from a syndrome, which is a collection of signs or symptoms that occur together. However, many conditions have been identified, yet continue to be referred to as "syndromes." Furthermore, numerous conditions of unknown etiology are referred to as "diseases" in many contexts. Refractory diseases do not respond to therapy by overcoming the resistance to drugs.

Illness, although often used to mean disease, can also refer to a person's perception of their health, regardless of whether they in fact have a disease. A person without any disease may feel unhealthy and simply have the perception of having an illness. Another person may feel healthy with similar perceptions of perfectly good health. The individual's perception of good health may even persist with the medical diagnosis of having a disease; for example, such as dangerously high blood pressure, which may lead to a fatal heart attack or stroke.

Pathology is the study of diseases. The subject of systematic classification of diseases is referred to as nosology. The broader body of knowledge about human diseases and their treatments is medicine. Many similar (and a few of the same) conditions or processes can affect non-human animals (wild or domestic). The study of diseases affecting animals is veterinary medicine.

Disease can be thought of as the presence of pathology, which can occur with or without subjective feelings of being unwell or social recognition of that state. Illness as the subjective state of "unwellness" can occur independently of, or in conjunction with, disease or sickness (with sickness the social classification of someone deemed diseased, which can also occur independently of the presence or absence of disease or illness (c.f. subjective medical conditions). Thus, someone with undetected high blood pressure who feels to be of good health would be diseased, but not ill or sick. Someone with a diagnosis of late-stage cancer would be diseased, probably feeling quite ill, and recognized by others as sick. A person incarcerated in a totalitarian psychiatric hospital for political purposes could arguably be then said to not be diseased, nor ill, but only classified as sick by the rulers of a society with which the person did not agree. Having had a bad day after a night of excess drinking, one might feel ill, but one would not be diseased, nor is it likely that a boss could be convinced of the sickness.

[edit] Transmission of disease

Some diseases such as influenza are contagious or infectious. Infectious diseases can be transmitted by any of a variety of mechanisms, including aerosols produced by coughs and sneezes, by bites of insects or other carriers of the disease, and from contaminated water or food (possibly by feaces or urine in the sewage), etc. When micro-organisms that cannot be spread from person to person might play a role, some diseases can be prevented with proper nutrition. Other diseases such as cancer and heart disease are not considered to be caused by infection. The same is true of mental diseases, which are not due to infection and are not able to transmit from person to person (i.e. anorexia).

[edit] Social significance of disease

The identification of a condition as a disease, rather than as simply a variation of human structure or function, can have significant social or economic implications. The controversial recognitions as diseases of post-traumatic stress disorder, also known as "Soldier's heart," "shell shock," and "combat fatigue;" repetitive motion injury or repetitive stress injury (RSI); and Gulf War syndrome has had a number of positive and negative effects on the financial and other responsibilities of governments, corporations and institutions towards individuals, as well as on the individuals themselves. The social implication of viewing aging as a disease could be profound, though this classification is not yet widespread.

A condition may be considered to be a disease in some cultures or eras but not in others. Oppositional-defiant disorder, attention-deficit hyperactivity disorder, and, increasingly, obesity, are conditions considered to be diseases in the United States and Canada today, but were not so-considered decades ago and are not so-considered in some other countries. Lepers were a group of afflicted individuals who were historically shunned and the term "leper" still evokes social stigma. Fear of disease can still be a widespread social phenomena, though not all diseases evoke extreme social stigma.

Sickness confers the social legitimization of certain benefits, such as illness benefits, work avoidance, and being looked after by others. In return, there is an obligation on the sick person to seek treatment and work to become well once more. As a comparison, consider pregnancy, which is not a state interpreted as disease or sickness by the individual. On the other hand, it is considered by the medical community as a condition requiring medical care and by society at large as a condition requiring your staying at home from work.

[edit] Metaphoric use

In addition, the term 'disease' is often used metaphorically for disordered, dysfunctional, or distressing conditions of other things, as in 'disease of society'.


[edit] References

  1. ^ Breman JG, Arita I, Fenner F. (2003). "Preventing the return of smallpox". New England Journal of Medicine 348 (5): 463-466. PMID 12556549. 
  2. ^ McVean G, Spencer CC, Chaix R (2005). "Perspectives on human genetic variation from the hapmap project". PLoS Genet 1 (4): e54. PMID 16254603. This review is free of charge
  3. ^ Skelding K.A., Gerhard GS, Simari RD, Holmes DR Jr (2007). "The effect of HapMap on cardiovascular research and clinical practice". Nat Clin Pract Cardiovasc Med 4 (3): 136-142. PMID 17330125. 

[edit] External links

Look up disease in
Wiktionary, the free dictionary.

[edit] See also

Retrieved from "http://en.wikipedia.org../../../d/i/s/Disease.html"