California's longstanding drought created ideal conditions for mosquitoes - and transmission of the West Nile virus.
Forty-five Californians died after contracting West Nile last year, up roughly 50 percent from 2014 and triple the annual average from the last 10 years, according to the latest figures from the state Department of Public Health.
The total number of human West Nile cases last year was 737, down about 7 percent from 2014 but nearly double the annual average from the prior 10 years.
Primarily spread by mosquitoes feeding on infected birds, West Nile causes few symptoms in the majority of people infected. But in a small percentage of cases, the reactions can be severe, even fatal. The disease usually peaks during summer, when mosquitoes are active and people tend to spend more time outdoors.
Just a few years ago, it looked like California health officials had West Nile in check, with severe cases increasingly rare. Public education campaigns reminded people to drain standing water and apply insect repellent. Government agencies eradicated hundreds of thousands of mosquitoes through pesticide spraying by truck and plane.
Then came 2012 and the start of the state’s protracted drought. That year, the number of reported human West Nile cases tripled from 158 to 479.
The highest human infection rates in 2015 were in Glenn County, an hour north of Sacramento. Nineteen people in the county of 29,000 tested positive for the virus, more than 30 times the statewide rate. Butte and Yuba counties also had high infection rates. All three counties have an abundance of rice fields, where standing water is a breeding ground for mosquitoes.
This map shows human West Nile infection rates by county in 2015.