Atlanta ? Health officials are investigating a never-before-seen form of the flu that combines pig, bird and human viruses and which has infected seven people in California and Texas. All the victims recovered, but the cases are a growing medical mystery because it’s unclear how they caught the virus.

None of the seven people were in contact with pigs, which is how people usually catch swine flu. And only a few were in contact with each other, according to the U.S. Centers for Disease Control and Prevention.

Still, health officials said it’s not a cause for public alarm: The five in California and two in Texas have all recovered, and testing indicates some mainstream antiviral medications seem to work against the virus.

Dr. Anne Schuchat of the CDC said officials believe it can spread human-to-human, which is unusual for a swine flu virus.