The victim's identity and details of her illness were not immediately available.

"This is an unfortunate incident," county Public Health Officer Wilma Wooten said in a statement. "Our thoughts are with the friends and family of this individual."

Wooten said it was "particularly important" for people with "underlying medical conditions" to promptly seek treatment from a physician if they experience flu symptoms.

She also reiterated health officials' call for common-sense precautions -- including covering the mouth when coughing, sneezing or nose-blowing, washing hands frequently and thoroughly, and staying home when sick -- to help prevent the spread of the disease.

"These routine precautions can go a long way in protecting your health and the health of those around you," Wooten said.

For additional information on H1N1 influenza visit www.sdcounty.ca.gov or www.cdc.gov or call 2-1-1 San Diego.