The U. S. Centers for Disease Control and Prevention reported on Thursday that seven people have been diagnosed with a flu that combines pig, bird and human viruses in California and Texas.

All the infected people have recovered but the mystery that is confounding everyone is how they caught the virus. None of the seven were in contact with pigs and a few were in contact with each other. In a telephone briefing the CDC's Dr. Anne Schuchat said, "We are likely to find more cases. We don't think this is time for major concern around the country."

Out of the people infected five were in California and two in Texas and only one of the seven was sick enough to need to be hospitalized. Tests that were conducted have indicated that some mainstream antiviral medications seem to work against the virus.

"We believe at this point that human-to-human spread is occurring," Schuchat said. "That's unusual. We don't know yet how widely it is spreading ... We are also working with international partners to understand what is occurring in other parts of the world."

The CDC's Dr. Nancy Cox said virus samples from the seven appear to carry genes from swine flu, avian flu and human flu viruses from North America, Europe and Asia. Mixes of bird, pig and human virus have been seen before, but never such an intercontinental combination with more than one pig virus in the mix.

"We haven't seen this strain before, but we hadn't been looking as intensively as we have," Schuchat said. "It's very possible that this is something new that hasn't been happening before."

A 10-year-old boy in San Diego County and a 9-year-old girl in neighboring Imperial County were the first to be detected with the virus and one was detected at a Navy clinic that participates in a specialized disease detection network, and the other was caught through a specialized surveillance system set up in border communities, CDC officials said.

Investigators on Thursday said they had found five more cases which included a father and his teenage daughter in San Diego County, a 41-year-old woman in Imperial County and two 16-year-old boys who are friends and live in Guadalupe County, Texas, near San Antonio.

CDC officials are trying to pin the origin of the virus and are looking into if it is connected with the unusually late and severe flu season to hit Mexico which resulted in the death of 20 people. In a statement Mexico's Ministry of Health said, "Generally the period of infection ends during the last week of February and the first week of March, but this year there was an atypical situation where the transmission period was prolonged until April."

The symptoms of swine flu are similar to those of the regular flu, mostly involving fever, cough and sore throat, though some of the seven also experienced vomiting and diarrhea. The CDC has said doctors must think about the possibility of swine flu when patients appear with flu-like symptoms, to take a sample and send it to state health officials or the CDC for testing.

Cox said the CDC is already preparing a vaccine against the new strain, just in case. "This is standard operating procedure," he said. U. S. health officials are consulting with Mexican and Canadian health officials, a CDC spokesman said.