A rare case of people being infected with both swine and seasonal flu has been documented in Cambodia, raising concern about the possibility of a potent combination strain, said a study out Wednesday.

The unusual diagnoses were made in a 23-year-old teacher and one of his young male students, who had H1N1 and a human season flu H3N2 at the same time, said the findings in the American Journal of Tropical Medicine and Hygiene.

Neither patient was hospitalized and their illnesses did not appear any more severe than in typical patients who are afflicted with a single strain.

The cases date back to 2009, the year the pandemic H1N1 flu emerged, and do not pose a current threat, but rather remind experts of the dangers that a strain such as H5N1 bird flu could mix with human flu and sicken millions.

"Influenza viruses are continually changing," said study author Patrick Blair, director of respiratory diseases at the US Naval Health Research Center in San Diego, California.

"Finding a co-infection in an area where there is considerable seasonal flu, pandemic flu and H5N1 avian flu shows there is an opportunity for co-mingling in swine or human hosts that could create an ominous global health problem."

In the Cambodian case, researchers analyzed and sequenced both virus genomes and found there had been no "genetic recombination," or mingling of the two.

Other case studies included in the report also show that such co-infections are rare.

One study in 2010 of 2,000 samples turned up no cases of dual infections and another pointed to fewer than two dozen co-infections with H1N1 -- one in Singapore, six in China, and 11 in New Zealand.

Infectious disease expert Peter Hotez, president of the American Society of Tropical Medicine and Hygiene, said the research provides more reason for world governments to "remain vigilant" and share information.

"Highly infectious strains of the virus against which humans have little defense can spread from one continent to another with 24 hours," he said.

The study noted that southeast Asia "has proven to be a critical region for the adaptation and emergence of variants of seasonal influenza viruses as well as an area of zoonotic virus transmission in humans."

Since 2005, the World Health Organization has counted 566 human infections with H5N1 avian flu and 332 deaths, most of them in the Near East and southeast Asia.

In Cambodia, where vaccination against the flu is rare, 16 of the 18 people infected with H5N1 flu have died, with the most recent fatal case in August.