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A disease is an abnormal condition of an organism that impairs bodily functions, ^[1] associated with specific symptoms and signs. ^{[2] [3]}

Jesus Navarro has a,"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, isolated symptoms, deviant behaviors, and atypical variations of structure and function, while in other contexts and for other purposes these may be considered distinguishable categories.^{[ citation needed]}

Classifying a condition as a disease is a social act of valuation, and may change the social status of the person with the condition (the patient). Some conditions (known as culture-bound syndromes) are only recognized as diseases within a particular culture. Sometimes the categorization of a condition as a disease is controversial within the culture.

Many different factors intrinsic or extrinsic to a person (or plant or animal) can cause disease. Examples of intrinsic factors are genetic defects or nutritional deficiencies. An environmental exposure, such as second-hand smoke is an example of an extrinsic factor. Many diseases result from a combination of intrinsic and extrinsic factors. For many diseases, many cause or set of causes has been identified.

There are many different factors that can cause disease. These can be broadly categorized into the following categories such as social, psychological, chemical, and biological. Some factors may fall into more than one category. Biochemical 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 protein gene that causes Huntington's Disease) and at the other extreme is caused entirely by environmental factors. Environmental factors include toxic chemicals (e.g. acetaldehyde in cigarette smoke and dioxins released from the breakdown of Agent Orange) and infectious agents (e.g. smallpox virus and poliovirus). In between these extremes genes (e.g. NOD2/ CARD15) and environmental factors (e.g. Gut microbiota) interact to cause disease, as seen for example in the inflammatory bowel disease Crohn's Disease (Fig 1, right).

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).

A large number of SNPs spaced throughout the genome have been identified recently in a large project called the HapMap project ^{[4] [5]}). The usefulness of the HapMap project and SNP typing and their relevance to society was covered in the 27 October 2005 issue of the leading international science journal Nature (journal).

A large number of genes have been identified that contribute to human disease. These are available 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.

Some diseases such as influenza are contagious and 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 faeces), etc. Also, there are sexually transmitted diseases. 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.

Living with disease can be very difficult. 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^{[ citation needed]}, attention-deficit hyperactivity disorder^{[ citation needed]}, and, increasingly, obesity^{[ citation needed]}, 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^{[ attribution needed]}. 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 one's staying at home from work.

This chart, compiled in 2002 from the global burden of disease study performed by the World Health Organization shows an overview of the impact of various classifications of disease, segregated by regions with low and high mortality:

File:GlobalBurdenOfDisease2002.PNG

Look up disease in Wiktionary, the free dictionary.

• Health Topics, MedlinePlus descriptions of most diseases, with access to current research articles.
• OMIM The best web resource for information on genes that cause disease at Online Mendelian Inheritance in Man
• NLM The most comprehensive database of top quality science information on the web at US National Library of Medicine
• Health Topics A-Z, fact sheets about many common diseases at Center for Disease Control
• GluvSnap.com, Latest disease and medical news
• The Merck manual containing detailed description of most diseases