A total of seven swine flu cases have been reported so far in California, indicating the potential for a swine flu pandemic, health officials warned on Sunday.

Four cases were reported in San Diego County and three in Imperial County, both in Southern California. The latest victim, a35-year-old Imperial County woman diagnosed April 4, has since fully recovered, according to the California Public Health Department.

Two other cases have been reported in Kansas and three in Texas.

What makes the outbreak different is that scientists believe the disease is now spreading through human-to-human contact, rather than human-to-pig contact, said the department.

California Governor Arnold Schwarzenegger has opened an emergency operations center, where public health officials are monitoring the progress of the virus and coordinating their response with federal health officials.

To avoid the disease, experts recommend washing hands frequently. If you are sick, avoid contact with others. So far, an effective vaccine has not been developed.

Symptoms include a fever of 100 degrees or more, vomiting, coughing, a sore throat and diarrhea.

No swine-flu-related deaths have been reported in the United States.

Containing the outbreak is unlikely, Dr. Anne Schuchat of the national Centers for Disease Control and Prevention in Atlanta said in remarks published by the Los Angeles Times on Sunday.

"Having found the virus where we have found it, we are likely to find it in many more places," she said. "It is clear that this is widespread, which is why we do not think we can contain spread of this virus."

The World Health Organization declared the outbreak a "public health emergency of international concern."

Dr. Margaret Chan, director-general of WHO, told The Times the flu "has pandemic potential," because it appears to be transmitted from human to human. But she noted that it was far too early to predict whether a pandemic would occur.

The situation is much worse in Mexico, where more than 1,300 people are believed to have been infected, and among them 81 people died.

Mexico City has been virtually shut down in an attempt to stop the flu's spread, and many of the city's 20 million people have taken to wearing surgical masks.