A fourth flu-related death in the Bay Area was reported Thursday out of San Francisco, according to health officials.
The death is one of nine confirmed fatal infections of the flu throughout the Bay Area this season.
The person died around the end of December after contracting the H1N1 strain, San Francisco Department of Public Health spokeswoman Colleen Chawla said.
A second flu-related death occurred earlier this week in Santa Clara County, public health department spokeswoman Amy Cornell said Thursday.
A 61-year-old man, who had underlying medical conditions, died sometime since Monday, only a few weeks after the death of a 41-year-old woman just before Christmas, Cornell said.
A 23-year-old Sonoma County resident died Wednesday after contracting the H1N1 strain of the flu, Sonoma County Health Service assistant director Tammy Moss Chandler said.
Also Thursday, health officials in San Mateo County said a woman in her 40s had died after contracting the H1N1 strain of the flu. She also had underlying medical conditions, according to San Mateo County Health System spokeswoman Robin Thaw.
Health officials are reminding all residents to protect themselves against the virus.
"There are also healthy people that are being hospitalized," Thaw said.
A death in Contra Costa County reported this month was confirmed to be flu-related earlier this week, according to health officials. The 48-year-old woman who had underlying health conditions died after she was infected with the H1N1 virus.
There have been 17 flu hospitalizations in that county so far this flu season.
In Alameda County, the first and only flu death of the season so far occurred the week of Dec. 22, county public health department spokeswoman Sherri Willis said.
In Marin County, there have been two flu deaths this season, health officials said.
A 63-year-old man with chronic medical conditions died on Dec. 27, and a healthy 48-year-old woman died of an influenza-related complication on Jan. 6, according to Marin County Public health officials.
The H1N1 influenza strain, known as "swine flu" when it first emerged in 2009, appears to be the main strain afflicting people this flu season.
The strain was particularly rampant during the 2009-10 flu season and affected children and young adults more than older adults, state health officials said.
Peak flu season is between January and March.
Health officials are urging vaccinations for everyone ages 6 months and older. This year's vaccine protects against H1N1.