The new H1N1 influenza virus bears a disturbing resemblance to the virus strain that caused the 1918 flu pandemic, with a greater ability to infect the lungs than common seasonal flu viruses, researchers reported on Monday.

Tests in several animals confirmed other studies that have shown the new swine flu strain can spread beyond the upper respiratory tract to go deep into the lungs -- making it more likely to cause pneumonia, the international team said.

"There is a misunderstanding about this virus," he said in a statement. "There is clear evidence the virus is different than seasonal influenza."

Writing in the journal Nature, Kawaoka and colleagues noted that the ability to infect the lungs is a characteristic of other pandemic viruses, especially the 1918 virus, which is estimated to have killed between 40 million and 100 million people.