DURBAN, South Africa—Former entomologist Diarmid Campbell-Lendrum of the World Health Organization worries about nosebleeds more than the average person. That's because he's one of the estimated 12 million people worldwide afflicted with leishmaniasis—a potentially fatal parasitic disease characterized most often by lesions on the skin and/or mucus membranes—caused by the bite of a sandfly.

As the team leader for climate change and health at WHO and an environmental epidemiologist, Campbell-Lendrum is also in a position to worry more about how global warming is going to affect such so-called vector-borne diseases. "Is climate change going to bring malaria back to the U.S. and Europe? It's not," he asserts. "Climate change is eroding the environmental determinants of health: water, food, increasing disease," he says. Already WHO research suggests that current warming of global average temperatures of just under one degree Celsius is responsible for an additional 150,000 deaths per year, largely due to agricultural failures and diarrheal disease in developing countries. "All the inputs are on the conservative side," says Campbell-Lendrum, who helped come up with the number.

As a result, WHO—and a consortium of other public health organizations—declared climate change to be among the most pressing emerging health issues in the world at the recent climate negotiations here in South Africa. Consider some of the changes that are already taking place: extreme heat waves, such as the one in Europe in 2003 that killed 46,000 people; changes in bacterial diseases due to water contamination and a quickening of bacterial growth rates in warmer temperatures; worsening levels of ground-level ozone, otherwise known as smog, which is responsible for worsening asthma and heart attacks (among other health effects); changes in pollen making allergies worse; changes in vector-borne diseases; as well as droughts, floods and other forms of extreme weather such as the 12 natural disasters in the U.S. this year that caused at least $1 billion in damage.

"Those things are with us now," says Dr. Hugh Montgomery of the U.K. Climate and Health Council, one of the groups calling for action to combat climate change—and to consider public health when doing so. "If we continue this script, we are writing a death certificate for humanity on our planet."

Taking public health into account would help in more ways than one. Climate change–fighting measures such as extending modern electricity to those who now burn wood, coal or dung indoors (thereby shortening their lives as well as contributing to global warming) would likely save as much money or more in public health costs as the price of implementing the new technologies. The alternative is to potentially see a reversal in the dramatic gains in the past century of public health of in some parts of the world. "Just as the HIV epidemic caused us to have a reversal in recent gains in public health in this country," notes Dr. Rajen Naidoo of the Nelson Mandela Medical School in Durban, "so too does climate change now."

In fact, something as simple as adding a solar hot-water heater to the roof of a village school can cut down on diarrheal disease in a country like South Africa. The hot water allows better hand-washing and thus better hygiene. Outfitting health clinics to cope with the various increased risks caused by climate change would also help, as would investments in climate-resilient infrastructure, such as better water-treatment facilities.

Public health infrastructure has its own role to play in combating climate change. For example, hospitals can curb solid waste by promoting breast milk over formula for infants, or simply cut down on nonessential energy use. The Taiwanese branch of a network of some 800 hospitals worldwide has set itself a target of reducing emissions by 13 percent below 2007 levels by 2020. "That's equal to reforesting 34 New York City Central Parks," says Dr. Chiou Shu-ti, director-general of Taiwan's Bureau of Health Promotion and vice chairperson of the International Network of Health Promoting Hospitals.

Ultimately, however, humanity could be on course for as much as 6 degrees C of warming by century's end. Such a temperature rise would put even larger swaths of the globe in danger of breaching high temperatures of 35 degrees C on a regular basis for extended periods of time—the point at which the human body's ability to thermally regulate itself breaks down. That may be the ultimate public health impact of climate change. "It would exceed the limits for habitability," says Sir Andy Haines of the London School of Hygiene and Tropical Medicine.

Or, regardless of leishmaniasis and other potential human health concerns, as Campbell-Lendrum puts it: "If [the highest temperature estimate] happens, all bets are off."