SAN DIEGO -- West Nile virus, a deadly disease carried by birds and mosquitos, has infected a man in the virus' first human strike in San Diego County this year.

The patient, from Los Angeles, contracted the disease outside the county and was hospitalized after falling ill in San Diego County earlier this month, county health officials said Thursday.

The infection is the first of many signs that the disease, which has killed thousands in other states but has been slow to reach the San Diego area, is gaining a foothold in the county.

Also Thursday, health officials announced that a third horse had been infected in the county, and six more dead birds tested positive for the virus. A total of 25 dead birds have been found with the virus this summer.

Health workers say the best way to keep the disease from infecting people and livestock is to avoid and get rid of mosquitos.

"It's important for the public to protect against (the virus) by taking precautions, including avoiding outdoor activity at dawn and dusk, and using insect repellent," San Diego County Public Health Officer Wilma Wooten said in a written statement.

Getting rid of puddles, pools and any standing water in which mosquitos could breed is another way of keeping the disease from infecting people and animals.

The county veterinarian encouraged residents with animals to get their horses vaccinated against West Nile virus, for which there is a horse vaccine but no human vaccine.

West Nile virus mainly affects birds but can be spread to humans, horses and other animals by mosquitoes that feed on infected animals and then people. It was discovered in the United States on the East Coast in 1999, but spread westward in the early 2000s, creating public health worries nationwide.

Most people who get infected don't get sick. But the virus can cause flulike symptoms, and in rare cases, it can cause serious brain swelling and lead to paralysis and death.

The county's office of environmental health has issued tips for avoiding mosquitos and slowing down the spread of the disease. They include:

- eliminating standing water around homes;

- using mosquito-eating fish, available free from the county, in pools, ponds and horse-troughs;

- wearing long sleeves and pants when outside, especially at dawn and dusk;

- stabling horses at dawn and dusk hours;

- wearing insect repellent; and

- maintaining window screens to keep bugs out.

More information about the virus can be found at www.sdfightthebite.com or by calling (888) 551-INFO.