With mosquito-borne illnesses specifically, it appears that there may be a delayed effect. In the short term, after a hurricane, there should actually be a lower risk of contracting these viruses, because the water likely washed away the existing breeding sites.

“But then over time, as the floodwaters recede, you’re left with pockets of water which are good for breeding both Culex mosquitoes and Aedes mosquitoes,” says Peter Hotez, the dean of the National School of Tropical Medicine at Baylor College of Medicine in Texas. Culex mosquitoes carry West Nile, as well as St. Louis encephalitis and Japanese encephalitis. Aedes aegypti are the primary carriers for Zika, as well as dengue, chikungunya, and yellow fever.

This is what happened after Hurricane Katrina in 2005. There was no increase in West Nile disease or St. Louis encephalitis in Louisiana or Mississippi that year. Researchers suspected this was because not only did flooding and winds destroy mosquitoes’ habitats, but evacuees ended up in areas less affected by those diseases.

However, the following year, researchers observed a more than twofold increase in cases of West Nile neuroinvasive disease—a more serious infection that can cause convulsions and even coma—in areas affected by Katrina. If Hurricane Harvey leaves a lot of standing water amongst the damage—standing water that stays for some time—Texas could see a similar delayed uptick in cases.

But it’s only August, and it’s possible the washed-away mosquitoes could still return this year. “The timing is kind of interesting,” Hotez says. “If we were at the end of arbovirus season, or we were headed well into the fall, then the effect would be beneficial because you’re going to wash the mosquitoes away and then basically, transmission season’s over. [But] we still have a lot of weeks of mosquito transmission season in Texas left.”

Robert Wright, the environmental health supervisor for Austin’s public health department, says that even inland in his city, he is seeing some water pooling in ditches and storm drains as a result of the hurricane. In areas of Texas where the flooding is less severe, there may still be areas of standing water that develop and attract mosquitoes. You’d expect to see “an increase in the mosquito populations in the weeks following the subsiding of the floods” in Houston, Wright says.

The WHO researchers also acknowledged this possibility. “Natural disasters do not usually cause an immediate increase in arboviral diseases,” they wrote. “However, if hurricanes strike early in transmission season, there could be a late increase in risk after vector and host populations are re-established.”

If populations do increase, “mostly those will be nuisance mosquitoes,” says Chris Van Deusen, a spokesman for the Texas Department of State Health Services. “However there is certainly a potential that we could see disease vectors in increasing numbers.” He notes that Harris County has a “very robust” mosquito-control program, and hopefully any disease carriers would be picked up in the course of normal mosquito surveillance.