The twentieth century was a landmark in the history of mankind as a result of the widespread control and eradication of infectious diseases that historically had been the scourge of humans. The advent and effective use of new drugs, vaccines, insecticides, treatment and prevention strategies during and following World War II reinforced public health programs already in place, and provided the tools needed to bring many of the worst diseases under control. Smallpox was eradicated using a mass vaccination strategy. By the late 1960s, the "war on infectious diseases" was declared won by leading experts in the field and by the Surgeon General of the United States (Patlak, 1996).
Unfortunately, the major successes in controlling infectious diseases in the 1950s and 1960s was followed by two coincident global trends that would have an impact on the dramatic re-emergence of infectious diseases in the waning years of the twentieth century. The first was the redirection of the resources that were once used to control infectious diseases to other public health priorities, such as the "War on Cancer" in the early 1970s. The perception that infectious diseases were no longer a problem led to decreased resources, widespread deterioration of public health infrastructure to deal with infectious diseases, and complacency among government and public health officials as well as the public (Smolinski etal, 2003). This trend included medical education with a de-emphasis on preventive medicine and a strong focus on curative medicine in medical schools. Today, training in preventive medicine is not included in the curriculum of most medical schools in the US.
The second trend was the sharply increasing and unprecedented rate of human population growth following World War II that has continued for 60 years. Increasing human numbers have been a principal factor leading to uncontrolled urbanization, changes in agriculture, land use and animal husbandry practices, and accelerated globalization, all of which have been major and inter-related drivers of the re-emergence of epidemic infectious diseases (Gubler, 1998a).
The first evidence of the re-emergence of infectious disease occurred in the 1970s, but the process greatly accelerated in the latter two decades of the twentieth century. Old diseases that were once effectively controlled began to reappear in epidemic form - for example, dengue, Japanese encephalitis, West Nile Virus, epidemic polyarthritis, yellow fever, measles, plague, cholera, tuberculosis, leishmani-asis, malaria, etc. In addition, numerous newly recognized diseases began to cause epidemics, such as HIV/AIDS, the hemorrhagic fevers (Marburg, Ebola, Lassa, hantavirus, Crimean-Congo, arenaviruses, dengue and yellow fever), avian influenza, Hendra and Nipah encephalitis, severe acute respiratory syndrome (SARS), Lyme disease, ehrlichiosis, and others. In addition to those factors mentioned above, resistance of bacterial pathogens to antibiotics, drug resistance in malaria parasites, insecticide resistance in mosquitoes, new medical technology (e.g. organ transplantation) and immunosuppression by drugs and disease (AIDS), and eco-logic encroachment by humans and animals have all played a role in the emergence/ re-emergence of infectious diseases as a global public health problem (Gubler, 1998a, 2002; Smolinski etal., 2003). In 2002, an estimated 26 percent of deaths worldwide were attributable to infectious and parasitic diseases (Fauci et al., 2005); 24 percent of the global burden of disease, as measured by disability adjusted life years (DALYs), was caused by infectious diseases (World Health Organization, 2004).
A unique feature of the twentieth century re-emergence of infectious diseases has been the rapid global spread of some infectious agents, such as SARS, avian influenza, West Nile Virus, and dengue. This global spread is tied directly to modern transportation and globalization, both of which are directly dependent on the major urban centers of the world. In the past 50 years the global human population has exploded, and nearly all of that growth has occurred in the cities of the developing world. Here, the majority of the urban population typically lives in substandard housing with no electricity, water, waste management, or sewage systems. This creates ideal conditions for increased mosquito-, rodent-, waterand food-borne infectious diseases, as well as for sexually transmitted and communicable diseases. The global airline network connects these cities, providing the ideal mechanism for transporting exotic pathogens to new geographic locations. This chapter reviews the contribution of urbanization, both directly and indirectly, to the twentieth century re-emergence of infectious diseases, focusing on dengue/dengue hemorrhagic fever as a case study.
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