The author of today's post, John Balbus, M.D., M.P.H., is Chief Health Scientist at Environmental Defense.

In the past when I gave talks about dengue fever, I'd say it was a problem in Mexico, but relatively rare over the border in Texas. I need to update my slides. Following an outbreak of dengue fever in Brownsville, Texas, health investigators found that 38 percent of the town was at risk for the most dangerous form of the illness.

This is a big deal, and global warming may well play a role.

Dengue fever is a mosquito-borne viral illness found in tropical and subtropical regions. The most dangerous form is dengue hemorrhagic fever (DHF), which results from a second infection in someone previously infected. DHF involves low platelet counts and bleeding, and can lead to shock and death.

In 2005 there were 25 hospital cases of dengue fever in Brownsville, Texas, and 16 of these were DHF. The 16 people with DHF must have previously contracted dengue fever, possibly without realizing it.

Since people with good underlying health may not have symptoms with a first infection, health investigators from the Center for Disease Control screened residents' blood for past infection. Disturbingly, they found that 38 percent of Brownsville residents had antibodies to dengue virus. All these people are at risk for DHF. This is not good news, to say the least.

Is the spread of dengue fever in Texas due to climate change? It's impossible to say for sure, but right now dengue fever is partly kept in check by low winter temperatures in Texas that kill the mosquitoes. Unusually warm winters associated with changing climate could lead to more severe outbreaks in the future. And with few specific treatments available, people in Texas will need to remove breeding places for mosquitoes to keep the dengue at bay.