Epidemiology is the study of the factors that cause or encourage<unk> s. Some<unk> s 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 factors for<unk> s. In the study of communicable and non-communicable<unk> s, 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<unk> s in a population, a condition known as a syndemic. Epidemiologists rely on a number of other scientific<unk> ciplines such as biology (to better understand<unk>  processes), biostatistics (the current raw information available), Geographic Information Science (to store data and map<unk>  patterns) and social science<unk> ciplines (to better understand proximate and<unk> tal risk factors). Epidemiology can help identify causes as well as guide prevention efforts. In studying<unk> s, epidemiology faces the challenge of defining them. Especially for poorly understood<unk> s, different groups might use significantly different definitions. Without an agreed-on definition, different researchers may report different numbers of cases and characteristics of the<unk> . Some morbidity databases are compiled with data supplied by states and territories health authorities, at national levels or larger scale (such as European Hospital Morbidity Database (HMDB)) which may contain hospital<unk> charge 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<unk> <unk>  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<unk> s 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<unk> . For example, if a person dies at the age of 65 from a<unk> , and would probably have lived until age 80 without that<unk> , then that<unk>  has caused a loss of 15 years of potential life. YPLL measurements do not account for how<unk> abled 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. The quality-adjusted life year (QALY) and<unk> ability-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<unk>  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 billion<unk> ability-adjusted life years were lost to<unk>  and injury. In the developed world, heart<unk>  and stroke cause the most loss of life, but neuropsychiatric conditions like major depressive<unk> order cause the most years lost to being sick.<unk>  category Percent of all YPLLs lost, worldwide Percent of all DALYs lost, worldwide Percent of all YPLLs lost, Europe Percent of all DALYs lost, Europe Percent of all YPLLs lost, US and Canada Percent of all DALYs lost, US and Canada Infectious and parasitic<unk> s, especially lower respiratory tract infections, diarrhea, AIDS, tuberculosis, and malaria 37% 26% 9% 6% 5% 3% Neuropsychiatric conditions, e.g. depression 2% 13% 3% 19% 5% 28% Injuries, especially motor vehicle accidents 14% 12% 18% 13% 18% 10% Cardiovascular<unk> 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%
Disease
Precancerous condition
Cancer
Psychopathology
Drug
Drug 
Cancer cluster
Suboptimal health