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Table 1 West Nile virus infection and weather for Cook and DuPage county from 2002 to 2008.

From: Local impact of temperature and precipitation on West Nile virus infection in Culex species mosquitoes in northeast Illinois, USA

YEAR Average mosquito infection
(MIR)
Percent Culex mosquito pools positive Number human cases illness Annual precipitation in cm
(difference from average)
Degree weeks at week 35 base = 22 C
2002    686 85.7 (-9.4) 30.0
2003    23 88.6 (-6.5) 15.2
2004 2.6 12% 28 88.2 (-6.9) 6.1
2005 4.5 19% 182 65.2 (-29.9) 31.2
2006 4.0 17% 129 117.8 (22.7) 25.7
2007 2.0 7.6% 53 100.1 (5.0) 17.9
2008 1.3 6.5% 9 126.6 (31.5) 12.5
  1. Average annual precipitation for the weather stations in the area was 95.1 cm using 30-year normal measures.