Two local people have been infected with West Nile virus — one in Miami County and one in Clark County — and mosquitoes in Greene County have tested positive for the disease.

Greene County crews are spraying for mosquitoes in Yellow Springs, and Greene County Combined Health District workers will keep trapping and testing mosquitoes, said Deb Leopold, environmental health director.

The virus has been found in only mosquitoes in Greene County. Statewide, 13 of 17 counties that conduct surveillance have found the virus in mosquitoes, including Montgomery County.

Nine people have tested positive for the disease and required treatment in Ohio, including three people in Cuyahoga County and one person each in Clark, Clermont, Lorain, Miami, Putnam and Sandusky counties, according to the Ohio Department of Health. Seven of those people required hospitalization.

Dry conditions around the state mean more Culex mosquitoes, which carry the West Nile virus, said Tessie Pollock, spokeswoman for the Ohio Department of Health.

This week alone, 163 mosquito pools collected tested positive for the virus, ODH records showed.

“We haven’t seen this concentration of West Nile virus since 2002, when we started trapping and testing for it,” Pollock said. “A lot of the reason we’re seeing it is the weather.”

Drought conditions are creating small stagnant pools of water that are perfect breeding grounds for the mosquitoes that carry the disease, she said. “They’re dense with gunk from dead leaves and trees and the mosquitoes just love that.”

In addition to the breeding grounds created by the drought, hot temperatures speed up the mosquitoes’ life cycle, so they mature and breed earlier, which means more mosquitoes, she said.

West Nile virus is spread through the bites of infected mosquitoes, which contract the disease when they feed on infected birds. In a small number of cases, the virus can be spread through blood transfusions, organ transplants, breastfeeding and from pregnant mothers to their unborn babies. It is not spread through casual contact.

Only about 20 percent of people who are infected with West Nile virus develop symptoms, according to the Centers for Disease Control and Prevention. The disease is usually mild, and symptoms can include swollen lymph glands, a skin rash, fever, headache, body aches and nausea or vomiting. Infection can be more serious, leading to encephalitis or meningitis. It can be fatal, though deaths are rare.