The most pronounced effects of climate change on vector-borne diseases such as malaria or dengue fever will undoubtedly occur where the diseases are newly introduced at the edges of the vector range and people have little resistance built up. In Africa, this will often be at higher elevations that were formerly too cold to support these diseases. Increasing numbers of malaria cases have already been reported in the highlands of Madagascar and Ethiopia as a result of warming; in Rwanda, record high temperatures and rainfall in 1987 brought malaria into the highlands where local residents had no immunity. These incidents have led public health officials to fear that relatively small increases in temperature from global warming could spread malaria into large urban centers such as Nairobi, Kenya, and Harare, Zimbabwe, that currently lie just outside of the malaria range  .
Under a similar scenario, malaria and dengue fever could spread into large swaths of the temperate zone where populations now lack resistance. Rough models of the spread of malaria affected by global warming show that malaria prevalence may increase by 50 million to 80 million cases per year with an associated 3