More people were infected and died of West Nile virus across California this year compared to last year, especially in Los Angeles County. That’s where the number of infections peaked in September, recent data show.

Statewide, there have been 25 deaths so far this year, or six more than all of last year, according to recent data from the California Department of Public Health. In addition, 454 people from across the Golden State were infected this year, a 17 percent increase from 2016.

Though the end of the season is approaching, more reports of infections and deaths are likely, public health officials said.

In Southern California, Los Angeles County seemed to bear the brunt of both infections and deaths. So far, 235 people have been infected by the disease and 17 people have died, including one person in Long Beach. The numbers surpass last year’s 153 cases and 6 deaths, according to the Los Angeles County Department of Public Health.

RELATED STORY: Long Beach reports first death this year from West Nile virus

Concerned about the rising spread of the disease, Los Angeles County Supervisor Janice Hahn and officials with the Los Angeles County Department of Public Health launched a campaign in September. Teams ventured out to distribute educational packets to more than 20,000 schools, houses of worship and community centers across the region. The goal was to encourage residents to wear insect repellent outdoors and to empty outdoor containers of any size that hold standing water, where mosquitoes tend to breed.

“The number of human cases with onset of illness through September is higher than the previous 5-year average,” Los Angeles health officials stated in the agency’s latest report.

MAP: Here’s where West Nile virus has been found in the Valley in 2017

Other Southern California counties also saw increases so far this year compared to 2016:

*Orange County has reported 34 people who have been infection this year compared to 32 last year. Three people have died.

RELATED STORY: Orange County confirms first West Nile virus death of season

*In Riverside County, there were 24 people infected so far this year, compared to 11 last year. There have been no deaths reported to the state.

RELATED STORY: 2 Riverside County residents get West Nile virus for first time this year

*San Bernardino County reported that 41 residents have become ill so far this year, compared to 8 last year. Two people have died.

RELATED STORY: San Bernardino County reports first West Nile Virus death for 2017

It’s unclear why there was so much infection, but there could be several contributing factors said Dr. Vicki Kramer, chief of vector-borne diseases for the state Department of Public Health.

“The level of West Nile virus fluctuates from year to year, region to region, because there’s so many factors, such as climatic features,” Kramer said. Los Angeles County saw warmer fall temperatures this year. Also, there was more rain across the state.

Known as a disease of wild birds, West Nile virus is spread to humans by the bite of an infected mosquito. Infection occurs between mosquitoes and birds. In rare cases, a person can be become ill through a blood transfusion.

Kramer said there are some years when the bird population is lower, or in some cases, more resistant to the virus. The virus doesn’t always spread during rainier seasons, she noted.

In 2014, a drought year, there were 801 people infected with West Nile virus and 31 deaths. The following year also saw drought. Less Californians were infected, but 53 people died. Kramer said that shows that the disease may flourish when there less water resources can stagnate. Mosquitoes and birds then compete for those resources, passing on the disease.

The state also has been monitoring the Zika virus, but has seen no local transmissions, even though the mosquito that carries the disease is in some California communities. Most people who have been infected with Zika have traveled out of the country, Kramer said. Unlike West Nile virus, Zika is passed from mosquito to person, then person to person through sex, or mother to unborn child.

Most people infected with West Nile virus do not become seriously ill, but some experience flu-like symptoms. About 1 percent of them can develop serious neurological illness. People who are 50 and older or who have pre-existing medical conditions such as diabetes or high blood pressure have the greatest risk of developing complications such as meningitis and encephalitis, health officials say.

Though West Nile virus has been in California for more than a decade, Kramer said the disease and the mosquitoes that carry it, continue to be a great risk to state residents.