Causes nof diseases

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, no 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 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 .

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


A large number of SNPs spaced throughout the genome have been identified recently in a large project called the HapMap project[1][2]). 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. WHO defines heatlh as a, State of physically,mentally,socially,economically and spiritually well being not merely the absence of disease or infirmity.