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Population growth and distribution and consumption patterns have been major driving forces of social and environmental changes in relation to land use, deforestation, agricultural practices, and water management. Research increasingly shows that many of these changes are linked to patterns of infectious disease.

Human migration, whether due to poverty, conflict, or climate-induced habitat changes, can foster the spread of emerging and re-emerging infectious diseases. Migration introduces diseases to new locations and exposes susceptible resident populations to new vector species. The devastating impact of infectious disease patterns was a common change of the initial contact of Native American groups and Pacific Islanders with Europeans. Modern transportation patterns are also having an impact. For example, the mosquito Aedes albopictus, which can breed in stagnant water in discarded tyres, has been globally distributed from Asia through transportation of used tyres on cargo freighters (Schaffner and others 2004, Madon and others 2002). The transfer of SARS in 2003, from South Asia to Toronto in Canada, could be traced to a single infected human who made the journey by commercial jet while incubating the disease (Figure 1). HIV/AIDS was spread widely throughout southern and central Africa by long-distance truckers, and globally by air travellers.

Figure 1: SARS cases and deaths, 2003

Source: WHO 2004

Unplanned rapid urbanization has resulted in inadequate housing and lack of water, sewer and waste management systems for large numbers of people in different parts of the world. When crowded human populations live in close association with large populations of mosquitoes, rodents, and other vermin, there is a dramatic increase in epidemics of diseases borne by water, food, mosquitoes and rodents, as well as in communicable diseases.

Aedes aegypti – the principal vector of dengue and yellow fever.
Source: David Scharf/Still Pictures

Urbanization has been the major driving force in the dramatic global resurgence of epidemic dengue and the re-emergence of its complication, dengue hemorrhagic fever (DHF) (Gubler 2004, Ko and others 1999). The global prevalence of dengue has grown dramatically in recent decades. Before 1970 only nine countries had experienced DHF epidemics: that number increased more than four-fold by 1995. It is now endemic in more than 100 countries, with South-east Asia and the western Pacific most seriously affected.

Some 2 500 million people are now at risk from dengue. In the 1950s an average of 908 DHF cases were reported to the World Health Organization (WHO) each year. This rose to an average of 514 139 cases a year for the period 1990-98. In 2001, there were more than 609 000 reported cases of dengue in the Americas alone, more than twice the number of dengue cases in 1995 (WHO 2004b).

In coastal areas, population pressure leading to coastal degradation have increased epidemics of waterborne diseases such as cholera. This may also have increased the impact of toxins resulting from algal blooms known as red tides.


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