In poor countries inadequate sanitation and garbage disposal, combined with poor resources at a local level mean there are numerous habitats for mosquitoes to breed in. From abandoned drainage ditches to discarded tires, buckets and bottles the problem is immense. In the absence of environment blanketing DDT the focus has turned to bednets - keeping mosquitoes (at least the nocturnal ones) from biting people.
In the developed world a new problem is arising - abandoned swimming pools. This problem has increased with the recent sharp rise in foreclosed and abandoned homes. In America the concern is not Malaria but West Nile virus. Fortunately such mosquito magnets can be detected remotely (although somewhat expensively) by the use of aerial surveys. Abandoned pools are generally green with algae. This is a regular tool in mosquito abatement in the Bay Area. For example, see this article today in the San Jose Mercury News:
The Santa Clara County Vector Control District will conduct an aerial search Monday for mosquito breeding sources that could potentially carry West Nile virus.
The district has made two previous aerial surveys this year, in early May and early August, which have led to more than 860 inspections and 135 treatments or fish plantings on local properties.
There's a longer article here about the problem in San Bernadino County. Remember the bottom line from this class - whenever anything changes in society there are new possibilities for disease transmission, potentially leading to outbreaks of new diseases or resurgences of old diseases.