Autumn is looming and warm weather may be headed south, but Chicago has not seen the last of the West Nile virus.

Researchers and the Illinois Department of Public Health said people should be very cautious of mosquitoes even though the seasons have changed.

The virus is on the decline,

peaking during the first week of September, but the threat of infection still exists, according to Edward Walker, a professor of microbiology and molecular genetics at Michigan State University and a lead researcher for the West Nile Virus Project in the Chicago area.

“Risk is very high right now,” Walker said. “It takes four to seven days after infection for symptoms to arise, so expect to see cases being reported in the next two to three weeks.”

The lag between incidence of infection and public health statistics is extended by the time needed to verify the illness as West Nile, according to Melaney Arnold, a spokeswoman for IDPH.

“It has to go through the lab and then back up to the state health department before the agency can publish a case report,” she said.

Marylin Ruiz, one of Walker’s

co-researchers and a professor of epidemiology at University of Illinois at Urbana-Champaign, why precautions must still be taken.

“I would say [for the] individual, the risk is not over,” she said. “As a population, we may see another 20 cases, and because it got so warm, I might revise that.”

According to IDPH daily reports and an Aug. 28 report from the Centers for Disease Control and Prevention, 2012 is one of Illinois’ worst years for reported West Nile cases since the virus first appeared in the human population in 2002.

According to IDPH, there have been 67 reported cases of West Nile Virus in Illinois this summer, with the vast majority of cases appearing in Cook and DuPage counties.

The virus appears to originate in the birds that mosquitoes feed on. The infected mosquito then bites humans, spreading the virus, according to Walker.

The northern house mosquito is the common name for the species of mosquito that is known to carry and transmit the virus. According to Ruiz, this particular mosquito is found primarily near humans.

“This species of [mosquito] likes people because we give them a place to lay their eggs,” she said. “You don’t find them out where you camp; they’re more likely to be seen around people’s houses.”

Arnold said that most cases are reported in urban areas because they offer a substantial breeding habitat for the insects.

“There are a lot of storm drainage systems because of the city streets, and those places are ideal for breeding,” she said. “More rural areas don’t have the extensive sewer system and are not as crowded, so the population isn’t sitting on top of the breeding grounds.”

The symptoms of a mild case of West Nile are similar to those of a common cold or flu, but the virus affects people in various ways, according to Ruiz. Sometimes it attacks the neurological system and can be deadly.

“You see the [incidence of] mortality in neurological cases, although people can have a very serious illness even if it isn’t neurological,” Ruiz said.

According to CDC case definitions, neurological cases are those in which the virus triggers a neuro-invasive disease, such as meningitis or encephalitis, which is characterized by such symptoms as a fever over 100 degrees or and a stiff neck, altered mental status, seizures and limb weakness.

Both the CDC and IDPH have listed preventative measures for limiting exposure to the virus. These include removing standing water in birdbaths and wading pools, applying bug repellent, repairing window screens and closing doors.

Risk of infection is particularly higher in September than July, according to Ruiz, which means more people will be outside enjoying cooler weather.