The huge amount of water dumped onto Texas by Hurricane Harvey flooded the Houston area and caused widespread damage, but there’s a new threat lingering weeks after Harvey’s devastation. The tremendous amount of water means that many stagnant pools have been sitting around for a while now, the perfect conditions for mosquitoes carrying potentially deadly diseases and viruses like West Nile and Zika to breed.

Authorities are surveying the state and conducting aerial and ground spraying to eliminate mosquitoes more than a month after Harvey made landfall back on Aug. 25. A total of 7 million acres will be sprayed, and authorities are cautiously optimistic about the results.

West Nile is a particularly big concern, as Texas already gets a ton of cases – the second most in the United States after California. West Nile is transmitted from birds to humans by the Culex mosquito. It doesn’t result in symptoms at first, but later may cause issues like fever, headache, diarrhea, vomiting, and in rare cases death.

“The rain left behind by Hurricane Harvey has created large areas where mosquitoes can lay their eggs,” reads a statement from the Texas Department of State Health Services. “To address increasing numbers of mosquitoes and the risk they pose to the recovery effort and public health, the Texas Department of State Health Services has activated its contract for aerial mosquito control and requested additional mosquito control assistance from the Federal Emergency Management Agency. Aerial spraying targeting mosquitoes will begin around dusk Thursday over Aransas, Bee, Nueces, Refugio and San Patricio counties, weather permitting.”