Tropical medicine is an interdisciplinary branch of medicine that deals with health issues that occur uniquely, are more widespread, or are more difficult to control in tropical and subtropical regions. Physicians in this field diagnose and treat a variety of diseases and ailments. Most infections they deal with are endemic to the tropics. A few of the most well-known include malaria, HIV/AIDS, and tuberculosis. They must be knowledgeable in the 18 lesser known neglected tropical diseases, which include Chagas disease, rabies, and dengue. Poor living conditions in underdeveloped tropical countries have led to a rising number of non-communicable diseases. These diseases include cancer and cardiovascular disease, which, in the past, have been more of a worry in developed countries. Physicians trained in tropical medicine must also be prepared to diagnose and treat these diseases. Training for physicians wishing to specialize in tropical medicine varies widely over the different countries. They must study epidemiology, virology, parasitology, and statistics, as well as the training required of an ordinary MD. Research on tropical diseases and how to treat them comes from both field research and research centres, including those of the military.