Changes in climate and weather may factor into some disease outbreaks, but it is not yet possible to determine whether global warming will actually cause diseases to spread, says a new report from the National Academies' National Research Council.

• Studies have shown that climate variation from one season or year to the next can affect the life cycle of many pathogens and disease-carrying insects, potentially affecting the timing and intensity of disease outbreaks.

• A number of computer models have been developed to simulate the effects of climate change on disease incidence, but estimates of the extent to which diseases will potentially spread have varied significantly among some of the models.

• In addition, observational and modeling studies generally are not able to consider complex social factors -- such as sanitation and public health services, population density, and travel patterns -- that also play important roles in disease dynamics.

Scientists know that weather can influence where and when some epidemics occur. For instance, mosquito-borne diseases such as dengue, malaria and yellow fever are generally associated with warm weather, while influenza epidemics usually occur during cold weather, and outbreaks of intestinal illnesses caused by cryptosporidiosis are linked to heavy rainfall.

However, basic public health protections such as adequate housing and sanitation, as well as the availability of vaccines and drugs, can limit the geographic distribution of diseases regardless of climate. For example, along the border between the United States and Mexico, dengue fever outbreaks are common just south of the Rio Grande River in Mexico, but are rarely seen in neighboring regions just north of the river in the United States.

Source: "Under the Weather: Climate, Ecosystems, and Infectious Disease," April 2001, Committee on Climate, Ecosystems, Infectious Diseases, and Human Health, Board on Atmospheric Sciences and Climate, National Research Council; "Impact of Climate Change on Human Health Remains' Highly Uncertain,'" ScienceNews.com.