Pigs, fingered as the source of so-called swine flu, can also be infected by humans, German scientists said on Thursday.
The findings add a new risk factor to the global pandemic sparked by the A (H1N1) virus, they said.
Virologists from the Friedrich Loeffler Institute infected five 10-week-old pigs with the human strain of virus and housed them with three uninfected pigs.
Within four days, the three pigs had become infected and all pigs were showing flu symptoms.
But five chickens which were housed, uncaged, with the pigs in the same room did not become infected, which eases fears that the virus can pass to poultry and pick up genes from avian microbes.
The study appears in the Journal of General Virology, published by Europe's Society for General Microbiology.
Its authors say the remarkably swift and easy way in which the three pigs became infected highlights the risk that the virus could become endemic in pig farms through people in close proximity.
Pigs are touted as a mix-and-match breeding vessels for dangerous new viruses, as they are able to simultaneously house human, avian and swine strains.
People do not have immunity to the H1N1 virus because it is new.
In its current form, though, the viral strain is relatively mild. The fear is that by mixing with other viruses, it could pick up genes that could make it more dangerous.
More than 98,000 cases of swine flu, including over 440 deaths, have occurred since the outbreak was first reported on April 24, according to the World Health Organisation (WHO).
The epicentre of the outbreak is in Mexico.
The new study points out, though, that no robust evidence has come to light to prove that pigs were to blame.
"So far, pigs or other animals have not been demonstrated to be involved in the epidemiology or spread of the novel influenza virus," it says.
"However, with the increasing numbers of of human infections, a spillover of this virus to pigs is becoming more likely."