LOS ANGELES, July 17 (Xinhua) -- Two men tested positive for West Nile virus, making them the first human infections in Southern California this year, health officials said on Thursday.

    The men, who live in Orange County near Los Angeles, recently donated blood and the virus showed up during a screening process, according to Orange County health officials.

    The patients, ages 23 and 41, were asymptomatic, meaning they never showed any symptoms of the disease, the officials said.

    "They're lucky," said Mike Hearst, Orange County Vector Control District communications director. "They're among the 80 percent who didn't even know they had it."

    Roughly 20 percent of people infected with West Nile virus show mild symptoms that may include fever, headache, body aches, nausea and vomiting.

    About one out of every 150 infected people will develop severe illness, with symptoms including convulsions, vision loss, numbness and paralysis. The symptoms can last for several weeks, with neurological effects that may be permanent.

    People over the age of 50 and those with certain medical conditions are at an increased risk of serious complications from an infection.

    Last year, 10 cases of West Nile virus in humans were reported in Orange County, one of which was asymptomatic. Statewide, there were 380 cases reported with 21 West Nile virus-related deaths.

    Health officials statewide are bracing for a tough West Nile season this year, with a high number of dead birds already testing positive for the disease.

    West Nile virus is carried in birds and transmitted through mosquitoes, which bite the birds and spread it to others, including humans.