West Nile virus has claimed its first death in California, evidence of the disease’s unrelenting march across America since entering the country through New York City five years ago.

State health officials say a 57-year-old Orange County man, who was being treated for encephalitis since mid-June, died on June 24.

“We knew that the virus had reached California, but this is the first real evidence of a more intense transmission. Now we basically have transmission of the West Nile virus from coast to coast,” Dr. Ned Hayes, a medical epidemiologist with the Centers for Disease Control and Prevention, told Reuters.

California health officials, who did not identify the man who died in June, said the disease had been detected for the first time in eight other counties and that 35 people across the state have tested positive for West Nile.

“The loss of this man is a sad reminder of the seriousness of West Nile virus,” Dr. Richard Jackson, California’s public health officer, said in a statement. “Although the risk of serious illness is low, I urge residents to take steps now to protect themselves from mosquito bites and this virus.”

West Nile virus was first detected in America in New York City in 1999, most likely arriving in an infected person or bird on an airplane from another country. Hayes said the virus most closely resembles a strain that had been circulating in Israel in 1998.

Since then, the mosquito-borne disease has spread quickly throughout the United States, resulting in serious outbreaks further and further west.

“While the virus has been moving across the country for the last 5 years the intensity has also moved westward,” Hayes said. “But there is continued transmission in eastern areas, as well, where it was first detected, so it looks like the virus is not going away, and it’s still something that needs to be dealt with for the foreseeable future.”

Hayes said that with elimination of the virus unlikely, the CDC was focused on trying to control its spread. Americans were advised to avoid being outdoors when mosquitoes were most active - generally in the two hours after dusk - to use insect repellent and to eliminate standing water.

About 80 percent of people infected with West Nile virus develop no symptoms. The remaining 20 percent suffer the acute onset of fever, headache and muscle and joint aches. About 1 percent of victims contract a severe neurological disease, such as meningitis or encephalitis.