West Nile Virus

Mosquitoes transmit West Nile virus. If infected with the virus, a person can develop a fever or mild headache. More severe infections are marked by a rapid onset of a high fever with head and body aches, disorientation, tremors and convulsions. In the most severe cases, paralysis or death may occur. Those who are 50 years of age or older are at the highest risk for serious illness. Symptoms typically occur within three to 14 days after the bite from an infected mosquito.

The best way to prevent West Nile virus and other mosquito-borne illnesses is to reduce the number of mosquitoes around your home and neighborhood and to take personal protection measures to avoid mosquito bites.

Projecting Human West Nile Virus

It is very difficult to project human West Nile virus infections from year to year as several variables, such as temperature, rainfall, and infection rates among birds and mosquitoes, are unknown.

Human West Nile Virus cases 2002-2020

Positive Results in the Area

Our surveillance helps to determine the risk for contracting the virus during the mosquito season. We track the number of positive results found in mosquitoes trapped in our surveillance activities.

View current surveillance results here

View positive test results from previous years

History of West Nile Virus in the United States

First Detected

In 1999, New York was affected by the first outbreak of West Nile virus. The disease then quickly spread across the country. According to the Centers for Disease Control, every state has reported West Nile virus activity, with the exception Alaska and Hawaii.

West Nile Virus in Illinois

The following timeline shows the history of West Nile virus in Illinois.


In 2002, Illinois experienced the worst West Nile virus outbreak in the country. There were a total of 884 human cases and 66 deaths that year. Cook County had 634 human cases in 2002 and 41 deaths.

2003 & 2004

Cooler temperatures in 2003 and 2004 are believed to have contributed to fewer human cases in Cook County. In 2003, Cook County had 20 human cases and one death. In 2004, Cook County had 21 human cases reported and two deaths.

2005 & 2006

Unfortunately the hot, dry summers of 2005 and 2006 brought an increase in human cases. There were 252 human cases in 2005 and 12 deaths and 215 human cases with 10 deaths in 2006.


In 2007, there were 101 human cases with four fatalities.

2008 & 2009

While 2008 and 2009 saw significant reductions in human illness.


2010, saw an increase in human cases. Of the 61 incidents of human infection, two occurred in our service area, one in Evanston and one in Northbrook.


The Illinois Department of Public Health reported 34 human cases of WNV occurred during 2011, thirteen in Cook County and three within District- Kenilworth, Glenview and Northbrook.

The Northbrook case was ruled a fatality due to West Niles virus.


The summer of 2012 was another year in which the area experienced a significant increase in West Nile virus activity. There were 290 human cases in Illinois, 173 in Cook County, 19 of which occurred within the service area of the NSMAD. While the state has reported 10 deaths due to WNV, none were residents of the area served by the NSMAD. Of those 19 human cases of WNV among district residents, 10 were in Evanston, three in Glenview, two in Morton Grove, one in Northfield, two in Skokie and one in Wilmette.