DES MOINES, Iowa (AP) – This time of the year is the peak season for two mosquito species most likely to carry the West Nile virus, which can cause high fevers and brain-swelling, an Iowa State University researcher said. Brendan Dunphy, an entomology researcher with the university, told The Des Moines Register that “we’re right in the thick of that time.”

Dunphy said the virus tends to be carried by the mosquito species, culex pipiens and culex tarsalis, and that late summer and early fall are the peak season for them. He said those species tolerate cool weather better than their cousins do. He said high numbers of mosquitoes buzzed into Iowa this summer, noting that in one week in July, special traps in Des Moines caught more mosquitoes than had been caught throughout the entire state in 2012. “The numbers were astronomical,” he said. “It was unbelievable.”

Overall, the bug numbers have fallen since July, but Dunphy said continued rainfall has helped the bugs continue to thrive in the state. “The bottom line is that water is the magic ingredient,” Dunphy said. Ann Garvey, deputy state epidemiologist, said that so far this year, the Iowa Department of Public Health has confirmed 11 illnesses and two suspected cases of the West Nile virus.

More than 40 Iowa residents were ill from the virus last year, which first appeared in the state in 2001. To avoid mosquito bites, Dunphy recommended repellents and emptying swimming pools and clogged rain gutters, where mosquitoes breed.