But many cities, especially in the Americas, have become infested with the Aedes aegypti mosquitos which transmit the disease and are once again at risk. Other diseases are re-emerging because the pathogen or the vectors that transmit them have become resistant to control measures. Although malaria has long been considered mainly a rural disease, it has become one of the most serious health problems in many urban centres where the anopheline mosquito can find standing water in which to breed. Dengue and dengue haemorrhagic fever, also spread by Aedes mosquitoes, are growing problems, the number of outbreaks and epidemics having increased significantly in the last decade. Some 2 billion people worldwide are at risk, mostly in urban areas.
Urbanization concentrates large numbers of people particularly vulnerable to infection.
The numbers of older people and of persons with HIV, two categories who are more vulnerable to many infectious diseases, are also increasing in towns. People are constantly moving in or out of cities, which can mean the arrival of newcomers who bring new infections to which the city population has little or no immunity. The rapid increases in travel and trade over the last few decades have increased opportunities for pathogens and vectors to spread to new areas. The mosquito responsible for transmitting dengue fever in Asia has become established in the United States, Brazil and parts of Africa. Anopheles stephensi , the principal vector for urban malaria, has adapted to the urban environment in India and the Eastern Mediterranean region, while other species have adapted to breed in swamps and ditches around urban areas in Nigeria and Turkey.