Editing Disease (section)

== Epidemiology ==
{{Main|Epidemiology}}
Epidemiology is the study of the factors that cause or encourage diseases. Some diseases are more common in certain geographic areas, among people with certain genetic or socioeconomic characteristics, or at different times of the year.

Epidemiology is considered a cornerstone methodology of public health research and is highly regarded in [[evidence-based medicine]] for identifying [[Risk factor (epidemiology)|risk factor]]s for diseases. In the study of communicable and non-communicable diseases, the work of epidemiologists ranges from [[outbreak]] investigation to study design, data collection, and analysis including the development of statistical models to test hypotheses and the documentation of results for submission to peer-reviewed journals. Epidemiologists also study the interaction of diseases in a population, a condition known as a [[syndemic]]. Epidemiologists rely on a number of other scientific disciplines such as [[biology]] (to better understand disease processes), [[biostatistics]] (the current raw information available), [[Geographic Information Science]] (to store data and map disease patterns) and [[social science]] disciplines (to better understand proximate and distal risk factors). Epidemiology can help identify causes as well as guide prevention efforts.

In studying diseases, epidemiology faces the challenge of defining them. Especially for poorly understood diseases, different groups might use significantly different definitions. Without an agreed-on definition, different researchers may report different numbers of cases and characteristics of the disease.<ref>{{cite news |url=https://www.nytimes.com/2011/03/08/health/research/08fatigue.html |title=Defining an illness is fodder for debate |author=Tuller, David |newspaper=The New York Times |date=4 March 2011 |url-status=live |archive-url=https://web.archive.org/web/20170103054656/http://www.nytimes.com/2011/03/08/health/research/08fatigue.html |archive-date=3 January 2017 }}</ref>

Some morbidity databases are compiled with data supplied by states and territories health authorities, at national levels<ref>{{cite web |url=http://www.aihw.gov.au/national-hospital-morbidity-database/ |title=National Hospital Morbidity Database |website=aihw.gov.au |publisher=[[Australian Institute of Health and Welfare]] | access-date=11 July 2013 |url-status=dead |archive-url=https://web.archive.org/web/20130813000437/http://aihw.gov.au/national-hospital-morbidity-database/ |archive-date=13 August 2013 }}</ref><ref>{{cite web |url=http://www23.statcan.gc.ca/imdb/p2SV.pl?Function=getSurvey&SDDS=3203&Item_Id=1724 |title=Hospital Morbidity Database (HMDB) |date=24 October 2007 |work=statcan.gc.ca |publisher=Statistics Canada |access-date=21 September 2015 |url-status=live |archive-url=https://web.archive.org/web/20160630153315/http://www23.statcan.gc.ca/imdb/p2SV.pl?Function=getSurvey&SDDS=3203&Item_Id=1724 |archive-date=30 June 2016 }}</ref> or larger scale (such as European Hospital Morbidity Database (HMDB))<ref>{{cite web |url=http://data.euro.who.int/hmdb/ |title=European Hospital Morbidity Database |archive-url=https://web.archive.org/web/20130902224516/http://data.euro.who.int/hmdb/ |archive-date=2 September 2013 |website=who.int |publisher=World Health Organization}}</ref> which may contain hospital discharge data by detailed diagnosis, age and sex. The European HMDB data was submitted by European countries to the World Health Organization Regional Office for Europe.

=== Burdens of disease ===
[[Disease burden]] is the impact of a health problem in an area measured by financial cost, mortality, morbidity, or other indicators.

There are several measures used to quantify the burden imposed by diseases on people. The [[years of potential life lost]] (YPLL) is a simple estimate of the number of years that a person's life was shortened due to a disease. For example, if a person dies at the age of 65 from a disease, and would probably have lived until age 80 without that disease, then that disease has caused a loss of 15 years of potential life. YPLL measurements do not account for how disabled a person is before dying, so the measurement treats a person who dies suddenly and a person who died at the same age after decades of illness as equivalent. In 2004, the [[World Health Organization]] calculated that 932 million years of potential life were lost to premature death.<ref name="WHODALY" />

The [[quality-adjusted life year]] (QALY) and [[disability-adjusted life year]] (DALY) metrics are similar but take into account whether the person was healthy after diagnosis. In addition to the number of years lost due to premature death, these measurements add part of the years lost to being sick. Unlike YPLL, these measurements show the burden imposed on people who are very sick, but who live a normal lifespan. A disease that has high morbidity, but low mortality, has a high DALY and a low YPLL. In 2004, the World Health Organization calculated that 1.5&nbsp;billion disability-adjusted life years were lost to disease and injury.<ref name="WHODALY">{{cite web |publisher=World Health Organization |website=who.int |url=https://www.who.int/healthinfo/global_burden_disease/estimates_regional/en/index.html |title=Disease and injury regional estimates for 2004 |url-status=dead |archive-url=https://web.archive.org/web/20101224055121/http://www.who.int/healthinfo/global_burden_disease/estimates_regional/en/index.html |archive-date=24 December 2010}} Standard DALYs (3% discounting, age weights). Also [https://www.who.int/entity/healthinfo/global_burden_disease/DALY14_2004.xls DALY spreadsheet] and [https://www.who.int/entity/healthinfo/global_burden_disease/YLL14_2004.xls YLL spreadsheet].</ref> In the developed world, [[heart disease]] and [[stroke]] cause the most loss of life, but [[neuropsychiatric conditions]] like [[major depressive disorder]] cause the most years lost to being sick.

{| class="wikitable sortable"
|-
! Disease category
! scope="col" style="width:10%;" | Percent of all YPLLs lost, worldwide<ref name="WHODALY" />
! scope="col" style="width:10%;" | Percent of all DALYs lost, worldwide<ref name="WHODALY" />
! scope="col" style="width:10%;" | Percent of all YPLLs lost, Europe<ref name="WHODALY" />
! scope="col" style="width:10%;" | Percent of all DALYs lost, Europe<ref name="WHODALY" />
! scope="col" style="width:10%;" | Percent of all YPLLs lost, US and Canada<ref name="WHODALY" />
! scope="col" style="width:10%;" | Percent of all DALYs lost, US and Canada<ref name="WHODALY" />
|-
| Infectious and parasitic diseases, especially [[lower respiratory tract infection]]s, [[diarrhea]], [[AIDS]], [[tuberculosis]], and [[malaria]]
| 37%
| 26%
| 9%
| 6%
| 5%
| 3%
|-
| [[Neuropsychiatric conditions]], e.g. [[major depressive disorder|depression]]
| 2%
| 13%
| 3%
| 19%
| 5%
| 28%
|-
| [[Injuries]], especially [[motor vehicle accidents]]
| 14%
| 12%
| 18%
| 13%
| 18%
| 10%
|-
| [[Cardiovascular disease]]s, principally [[heart attacks]] and [[stroke]]
| 14%
| 10%
| 35%
| 23%
| 26%
| 14%
|-
| [[Premature birth]] and other [[perinatal]] deaths
| 11%
| 8%
| 4%
| 2%
| 3%
| 2%
|-
| [[Cancer]]
| 8%
| 5%
| 19%
| 11%
| 25%
| 13%
|}