"We believe that human-to-human spread is occurring. That is unusual," Schuchat said at a news conference. "We don't know yet how wide it is spreading. We have taken steps to learn more."
All seven of the people with confirmed swine flu recovered. Only one patient was ill enough to require hospitalization. The swine flu disease was similar to human flu, except that nausea and diarrhea were more common.
Patients ranged in age from 9 to 54 and included three girls and four males.
Five of the people who got the virus live in the San Diego area and two live near San Antonio. A father and daughter in San Diego and two 16-year-old boys from the same school in San Antonio are among the cases. But the three other cases are not connected.
The San Antonio cases have no connection to one infected San Diego boy who traveled to Dallas while ill. None of the seven cases appears to have had direct contact with pigs.
"We are seeing a virus that appears to be spreading from person to person in humans, and the viruses have very similar genetic characteristics to each other," CDC flu division chief Nancy Cox, PhD, said at a news conference.
It's a "very interesting" virus, Cox said. The viruses from San Diego and San Antonio are not exactly identical, but are both a type A H1N1 swine flu virus with unusual characteristics.
Genetic analysis shows that it contains gene segments from North American swine viruses, from North American bird flu viruses, from one human flu virus, and from swine flu viruses seen in Asia and Europe but not previously known in the U.S.
It's not yet clear whether the virus was detected along the Mexican border because of an intensive respiratory-disease surveillance program or because of any link to Mexico.
The Public Health Agency of Canada today issued a warning to Canada residents returning from Mexico about a severe respiratory disease, possibly linked to flu, in parts of south and central Mexico.
While that illness seems more severe than the one seen in the swine flu patients, a CDC spokesman tells WebMD that the agency is in contact with health authorities from Mexico and the Pan American Health Organization, and is collecting swab samples from Mexican patients for testing.
At this time, there's no evidence that an unusual number of flu cases are appearing in either California or Texas as the relatively mild 2008-2009 flu season winds down. But the CDC has recovered virus from patients and has begun the process of developing a vaccine.
"We don't think this is a time for major concern," Schuchat said.
The virus can't be caught from eating pork or pork products.
The CDC suggests that people who live in or visit the San Diego or San Antonio areas should see a doctor or nurse for testing if they come down with flu-like symptoms. And they suggest that anyone who has a respiratory illness with fever should stay home to avoid infecting others.