Autopsies on people who have died from the new pandemic H1N1 flu show this virus is different from seasonal influenza, even if it has not yet caused more deaths, experts told a meeting.
Americans who died from swine flu had infections deep in their lungs, Sherif Zaki of the US Centres for Disease Control and Prevention told a meeting of flu experts, including damage to the alveoli - the structures in the lung that deliver oxygen to the blood.
This in turn caused what is known as acute respiratory distress syndrome - an often fatal development that leaves patients gasping for breath.
The World Health Organisation has confirmed 3,205 deaths globally from swine flu but experts agree all estimates of the extent of the pandemic are grossly understated because so few patients are ever actually tested.
Seasonal flu kills, too - about 250,000 to 500,000 cases a year globally, according to the WHO. But not in the same way as swine flu, which unlike seasonal flu frequently causes severe disease in young adults and children.
"It is very rarely you see what we call diffuse alveolar damage in fatal seasonal influenza," Dr Zaki told a meeting sponsored by the US Institute of Medicine, which advises government on health matters.
Seasonal flu causes bronchitis and other upper respiratory disease.
But Dr Zaki, the chief infectious disease pathologist at CDC, said the new virus had burrowed into the lungs of the 90 or so people he examined after they died, and they had huge amounts of the virus in their blood.
"This is almost exactly what we see with avian flu," added Dr Zaki. "This looks like avian flu on steroids."
Yoshi Kawaoka of the University of Wisconsin said tests in monkeys showed the virus lives and replicates 1,000-fold better in the lungs than does seasonal flu.
He said the No. 1 drug of choice against H1N1 - Roche AG's and Gilead Sciences Inc.'s Tamiflu - lowered the so-called viral load of virus in the lungs just enough to help the body fight back.
Experimental flu drugs lower it even more, notably Daiichi Sankyo Co., Ltd's CS 8958 and another drug called T-705 or favipiravir, made by Fujifilm Holdings Corp unit Toyama Chemical Co, Dr Kawaoka said.
Dr Zaki said 90 per cent of the fatalities he looked at had some condition that would predispose them to serious disease. They had a median age of 38 and one victim was a two-month-old infant who died within a day of getting sick.
Nearly half - 46 per cent - were obese, many had fatty liver disease, 27 per cent had heart disease and 22 per cent had asthma, he said.