AD Quality Auto 360p 720p 1080p Top articles1/5READ MORERose Parade grand marshal Rita Moreno talks New Year’s Day outfit and ‘West Side Story’ remake The number of cases in the county this year is down significantly from last year, which saw 309 people infected. Also, 12 people died last year and no one has died this year in the county. “What we’ve seen across the country is that places that have a very heavy year tend not to have a heavy year the next year,” said Dr. Jonathan E. Fielding, the county health officer. “Nobody really knows the answer why this happens, whether it’s a decline in the susceptible bird population, certainly the good job vector control does or education efforts. But I think there are factors beyond those we don’t understand yet.” Stephanie Heintz, spokeswoman for the Greater Los Angeles County Vector Control District, agreed the downward trend in the county involves several factors, including a warmer summer, more intensive education efforts and increased mosquito abatement efforts. “It could have been a little bit of everything,” Heintz said. “There is a theory that people gain an immunity to the virus. Maybe last year, there were a lot more cases than were reported and people gained an immunity. Maybe that’s why we didn’t have as many cases reported this year.” In the six years since the virus was first diagnosed in New York and spread across the country, the virus has demonstrated dramatic fluctuations in both the number of people infected and horses, birds, sentinel chickens and mosquito pools. Experts say these fluctuations are due to changing temperatures, rainfall and humidity, bird reservoirs, mosquito populations and efforts by mosquito abatement districts to kill mosquitoes. One reason for the drop in the number of infections in the county since last year could be decreases in local mosquito populations. And from March through May of this year, the season was much cooler compared with last year. As a result, mosquito increases normally seen in late spring were delayed. Also, record rainfall this year created larger-than-normal bodies of water and diluted nutrients, which could have slowed mosquito development. Another possible explanation is the large numbers of crows that died last year from the virus. The virus is transmitted to people and animals through mosquito bites. About 80 percent of those who are infected have no symptoms, but in some cases people can suffer paralysis and encephalitis. The elderly and those with weakened immune systems are at greatest risk for the illness. — Troy Anderson, (213) 974-8985 [email protected] local news?Sign up for the Localist and stay informed Something went wrong. Please try again.subscribeCongratulations! You’re all set! Although the third season of the West Nile virus was supposed to be milder, California saw cases rise to 910, one-third of the 2,733 cases diagnosed nationally, health officials said Thursday. A total of 18 Californians died this year, compared with 27 last year, according to a report released by Los Angeles County. But the number of people infected increased from from 779 last year, which was widely expected to be the state’s worst year. Most of those cases were in Southern California. Although the vast majority of people infected this year were in the Northern and Central parts of the state, Los Angeles County did have 43 cases, including 18 in the San Fernando Valley. That was followed by 16 cases in the San Gabriel Valley and eastern part of the county and six in the South Bay.