Drought and high temperatures don’t just threaten our water supplies and crops. They may help to spread disease. The recent outbreaks of West Nile virus across the United States in the midst of the past summer’s drought are a preview of how a changing climate could threaten our health.

Many mosquito-borne diseases are known to be sensitive to climatic conditions, including increased temperature, among other factors. Now there are indications that climate variability, not just higher temperatures, can also contribute to increased disease. While other factors come into play, West Nile virus outbreaks have been related to a combination of heat and drought followed by heavy downpours.

And that kind of weather pattern, according to The Intergovernmental Panel on Climate Change, is likely to occur more often with global warming. When West Nile virus first emerged in North America, conditions were ideal for an outbreak. The United States had never had a documented case of West Nile before it was introduced from overseas in 1999. That year, New York City experienced its driest and hottest spring and summer in a century. Stagnant and polluted pools of water throughout the city became the perfect breeding grounds for the mosquito vector, Culex pipiens.

Mosquitoes fed on birds, drawn to the shrinking pools of water, infecting them with West Nile virus and killing many of them in the process. Uninfected birds flew to wetter habitats as the drought deepened and mosquitoes within the city then had to turn to humans for their next blood meals. The heavy downpours that eventually broke the drought that summer created new breeding sites. The mosquito populations soared and the virus spread. At the end of the epidemic, more than 8,000 people had been infected, 62 had fallen ill to the virus and seven died.

This year, epidemiologists have again seen West Nile virus outbreaks in association with drought and heavy downpours. For example, Southeast Louisiana, where many cases of West Nile virus have occurred, and parts of Texas, saw less than half the normal rainfall in the first five months of the year and experienced dramatic downpours in July. The high summer temperatures increase mosquito-biting frequency and the infectiousness of each bite.

To date, close to 2,800 people have been diagnosed with West Nile virus in 35 states (and the District of Columbia) and 148 have died, according to tracking by the Centers for Disease Control and Prevention. New cases continue to be identified, and the number of cases has risen sharply since early August.

And the West Nile virus is here to stay. That’s because the disease, like other encephalitis-type viruses such as St. Louis Encephalitis and Eastern Equine Encephalitis, can remain in bird or animal populations indefinitely and we don’t yet have a vaccine for humans.

While some ecological conditions that may limit mosquito populations could occur, drought, high heat or other unusual conditions that lead to either booms in the infected mosquito population or declines in birds or other primary hosts will increase the likelihood of outbreaks in humans. These are climate conditions that are likely to become more and more frequent if global warming continues unchecked.

It is sobering to realize that pollution from greenhouse gases may be a contributor to this lethal public health threat. Early this summer, an interagency report by the Bush administration acknowledged that global warming is occurring, and that human activity – the pollution from cars and trucks as well as coal-fired power plants – is a primary culprit. Still, the Bush administration has failed to provide a meaningful plan to address the problem. It is time to acknowledge the connections between climate change and increased disease risks. It is time to take action to reduce greenhouse gas pollution.

Dickson Despommier is professor of Public Health and Microbiology at Columbia University. Janine Bloomfield is a senior scientist at Environmental Defense, a New York City-based nonprofit organization. For more information visit www.environmentaldefense.org