Tropical diseases, neglected diseases, diseases of the developing world (DDW), diseases of poverty: these are all different names to identify a heterogeneous group of infectious diseases produced by viruses, bacteria, protozoa, and worms, which disproportionately affect people in low and medium income countries. The term "tropical" refers to diseases that occur solely, or principally, in tropical areas, such as malaria, leishmaniasis, Chagas disease, African trypanosomiasis, schistosomiasis, onchocerciasis, lymphatic filariasis, and dengue . "Neglected" means that, in spite of the high mortality and/or morbidity produced by the disease, little scientific attention and resources have been dedicated to understanding and curing them . The meaning of the term "neglected" is exemplified in the study of Trouillier et al.  who showed that of the 1993 new chemical entities marketed between 1975 and 1999, only 16 were for tropical diseases and tuberculosis. Neglected diseases include those caused by Helminths (e.g., schistosomiasis, onchocerciasis, lymphatic filariasis, hookworm, ascariasis, and trichuriasis); protozoa (malaria, kinetoplastids, plus amoebiasis, giardiasis); bacteria (tuberculosis, leprosy, Buruli ulcer, trachoma, and bacteria causing diarrhea), and viruses (yellow fever, Japanese encephalitis, chikungunya, and viruses causing diarrhea). The name "diseases of the developing world" includes all the above and refers to those that disproportionately affect low-and medium-income countries. The three most devastating diseases, HIV/AIDS, malaria, and tuberculosis (TB), collectively called "the big three", are now high on the international agenda and receive more attention and funding. Frequently, these diseases are classified as "the big three", on one side, and as neglected tropical diseases (NTDs), on the other. Nevertheless, in spite of the increased awareness and financial efforts by the Global Fund and other funding organizations, drugs for the treatment of malaria are still not available in all the endemic areas, and the drugs for HIV/AIDS and TB are still largely inaccessible in Africa and south/southeast Asia .
On their web pages, both WHO (http://www.who.int/neglected_diseases/dis-eases/en/) and the Special Programme for Research and Training in Tropical Diseases (TDR) (http://apps.who.int/tdr/svc/diseases); established in 1975 by the UNICEF, the United Nations Development Programme, the World Bank, and the WHO, to focus on neglected infectious diseases) include similar but not identical lists of neglected and tropical DDW. For the sake of clarity, we will use the terms "tropical" and "neglected" with the meanings given above, and when we refer to all of them we will use the term "DDW". HIV/AIDS will only be discussed in this chapter when it affects the course of other DDW or its treatment.
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