Three new cases were found in California and two in Texas, bringing the total number of confirmed cases to seven, officials at the federal Centers for Disease Control and Prevention in Atlanta said. The CDC announced Tuesday that two children had been infected in San Diego. The agency has launched an investigation to try to determine how widespread the virus is.

Officials said there was no reason for alarm despite the growing number of cases, but they urged doctors to be on the look-out for more cases and said they were intensifying their efforts. They have also taken preliminary steps to create a vaccine against the virus if necessary.

"We don't think this is time for major concern around the country," Anne Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases, told reporters in a telephone briefing.

"We are taking steps to know more and stay on top of the situation," she said, adding that the agency had alerted the World Health Organization and the Pan American Health Organization.

Two of the new cases involved a father and a daughter in San Diego County, and two others were boys who attend the same school near San Antonio. Those cases, combined with the lack of evidence that any of the infected people had contact with pigs, led officials to believe that the virus was spreading from one person to another, Schuchat said.

"We believe at this point that human-to-human spread is occurring. That's unusual. We don't know how widely it is spreading," she said, adding that she expects more cases to be identified in coming days as the investigation unfolds.

The cases so far involve three females and four males ranging in age from 9 to 54. The first child became ill March 28. The CDC confirmed the three new California cases Wednesday and the two new Texas cases -- in 16-year-old boys -- yesterday. Laboratory testing showed that the virus does not match any known flu strains.

The infection has caused typical flu-like symptoms, including high fever, sore throat and cough, as well as vomiting and diarrhea; one person was hospitalized. But so far, the virus does not appear to be causing serious illness, Schuchat said.

"The good news is all seven of these patients have recovered," she said. The virus appears to be resistant to two drugs normally used to treat the flu, but two others appear to be effective against it.

Genetic analysis of the virus indicates it is highly unusual: It is a hybrid that resulted from a combination of four different viruses -- one that typically infects people, one that originated in North American birds and two from pigs in Europe and Asia.

"This combination has not been recognized before in the U.S. or elsewhere," Schuchat said.

Although the cases all are scattered along the U.S.-Mexico border, officials have not identified any cases in Mexico. But they are continuing to investigate.