Malaria, diarrhea, malnutrition and floods cause an estimated 150,000 deaths annually, with Asia accounting for more than half, said regional WHO Director Shigeru Omi.
Malaria-carrying mosquitoes represent the clearest sign that global warming has begun to impact human health, he said, adding they are now found in cooler climates such as South Korea and the highlands of Papua New Guinea.
Warmer weather means that mosquitoes' breeding cycles are shortening, allowing them to multiply at a much faster rate, posing an even greater threat of disease, he told reporters in Manila.
The exceptionally high number cases in Asia of dengue fever, which is also spread by mosquitoes, could be due to rising temperatures and rainfall, but Omi said more study is needed to establish the connection between climate change and that disease.
In Geneva, WHO chief Margaret Chan said the reality of climate change "can no longer be doubted. The effects are already being felt."
She cited climate-sensitive diseases such as dengue and cholera, which are currently present in South America and Angola, respectively, because of flooding. She also called for more progress in battling malaria.
Chan said the issue of health and climate change should be addressed by leaders of the Group of Eight developed nations at their meeting in Japan next month. "Not addressing the climate change impact on health would derail their previous investment in supporting countries in development," she said.
In the Marshall Islands and South Pacific island nations, rising sea levels have already penetrated low-lying areas, submerging arable land and causing migrations to New Zealand or Australia, Omi said.
He said poorer countries with meager resources and weak health systems will be hit hardest because malnutrition is already widespread, with the young, women and the elderly at particular risk.
Omi said unusual, unexpected climate patterns - too much rain or too little - will have an impact on food production, especially irrigated crops such as rice, and can cause unemployment, economic upheavals and political unrest.
Omi said governments need to strengthen current systems providing clean water, immunizations, disease surveillance, mosquito control and disaster preparedness.