(CBS) – With almost a month left until the start of the fall season, the Oakland County Health Department is advising caution because mosquitoes may be carrying West Nile virus.

“The West Nile virus mosquito is most active at dusk and dawn, so we ask people to exercise extreme caution during these times,” said Mark Hansell, director of the Oakland County Health Department.

Oakland County health officials say Three birds and the county’s first mosquito pool this year tested positive for the West Nile virus.

The summer season is right on schedule with a rise in cases, as experts say the heat favors the spread of the virus.

“They need water to reproduce, so do whatever you can to prevent standing water on your property, whether it’s cleaning the birdbaths on your property,” Hansell said.

In some cases, the virus can be fatal, especially for older adults. But with school starting soon, there is a risk of transmission to children.

“There are nighttime band practices or outdoor sports activities and people want to be outside, but that also increases the likelihood of transmission of these viruses in the fall,” said Edward Walker, professor of entomology at Michigan State University.

People cannot transmit the virus to each other; it is only transmitted through mosquito bites. Homeowners are advised to secure openings in their homes to prevent the insects from entering.

“We still observed mosquito bites on Halloween night,” Walker said.

Experts say the prolonged mosquito season has encouraged transmission of the virus. Hansell says animals other than birds are also at risk.

“We have cases of dogs getting West Nile virus. Deer populations are frequently tested for West Nile virus,” Hasell said.

Wearing mosquito repellent is the best way to avoid infection, and mosquitoes are being tested for the virus across the state to prevent its spread.

“I call them mosquito control programs because they are not just spray trucks, but a series of approaches that try to control the mosquito population in an integrated system,” Walker said.

According to the CDC, nearly 80% of people infected with West Nile virus do not develop any symptoms. Some have mild fever and/or nausea, while those over 50 may experience a more severe reaction.