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Epidemiology is the study of patterns of diseases and the factors affecting its spread. Incidence, prevalence and mortality for a disease may be determined. Collecting information on the distribution of disease helps to identify the underlying causes and if it turns out to be infectious, may point to how it is transmitted.
Other causes such as the links between smoking and lung cancer may also be determined. The data on morbidity (numbers ill) and mortality (numbers who have died) for a disease when expressed in certain ways enables comparisons across cities or countries to be made.
Statistics can be used to monitor the effectiveness of health provisions by both governments and the World Health Organisation (WHO).
Data collected by WHO shows that in developing countries, the main cause of death is infectious diseases, while in developed countries, very few deaths are caused by pathogens.
The reasons for this is that the incidence of infectious disease is low in developed countries due to vaccination, good standards of hygiene and nutrition, and the availability of antibiotics if they do contract one.
Although globally health is improving, there are still many poor people in the world and poverty is responsible for the highest death rates in some countries. In the richer countries, degenerative diseases are the biggest killers. This is mainly due to lifestyles that put people at risk of heart disease and cancers.