Swine flu has killed at least 500 people
H1N1 swine flu attacks the respiratory system in a more sustained way than the standard seasonal virus, research in animals shows.
Tests showed swine flu multiplies in greater numbers across the respiratory system, and causes more damage.
And instead of staying in the head like seasonal flu, it penetrates deeper into the respiratory tissues - making it more likely to cause pneumonia.
The University of Wisconsin study appears in the journal Nature.
It also suggests that swine flu may mimic the flu virus which caused the great pandemic of 1918, in which millions died.
The 1918 virus also had a greater ability than standard flu to cause damage to the respiratory system.
The researchers carried out their work on ferrets, monkeys and mice.
They also analysed samples taken from people who survived the 1918 pandemic and found that they seem to have extra immune protection against the current virus - again suggesting similarities.
However, the Wisconsin team stressed that swine flu produced, in the vast majority of cases, only mild symptoms, and is still sensitive to anti-viral drugs.
Professor Ian Jones, a flu expert at the University of Reading, said the latest study provided the complete analysis of the swine flu that researchers had been waiting for.
He said: "For a number of measures it shows that the new virus is more serious than seasonal H1N1 but that, nonetheless, the major outcome to infection is recovery.
"For the few cases of severe infection the data should help in the clinical management of hospitalised patients.
Professor Wendy Barclay, an expert in virology at Imperial College London, said: "It must be borne in mind that typical circulating human strains of H1N1 have been associated with rather mild illness in recent years, and that the swine origin H1N1 may be behaving in these animal models more alike the type of H3N2 viruses that caused a pandemic in 1968."
The World Health Organization has recorded nearly 100,00 cases of swine flu worldwide, although the true figure is likely to be much higher. The virus is known to have killed at least 420 people.