The Minnesota Department of Health says the number of human cases of West Nile virus in the state this year has increased to 27, up from 20 on Monday.
The mosquito-borne virus has caused one death in the state.
Health Department epidemiologist Dave Neitzel said the hot summer is one reason case numbers are running well ahead of recent years.
"Warm weather usually equals higher West Nile virus risk," said Neitzel. "Years like this year, where we had kind of an early warm-up and it stayed warm, that warm weather allows the virus to grow more quickly in mosquitoes."
Neitzel said avoid outdoor activity if possible at dusk and dawn, generally the time when mosquitoes are most likely to be encountered. He said applying insect repellant is another good idea.
"We recommend repellants that contain DEET," said Neitzel. "Up to 30 percent DEET is a very effective repellant."
Neitzel said the virus is most often found in agricultural areas, and most of the state's cases are in western and southern Minnesota. He said hot temperatures kept mosquito numbers high, even though water available for mosquito breeding may have been reduced.
West Nile cases are also climbing rapidly across the U.S. More than 1,100 have been reported so far, resulting in 41 deaths.