MIDWAY — The good news about the current outbreak of the so-called swine flu is it passes fairly easily from person to person but has a low fatality rate, one of the world's top virologists said Tuesday.

That's also the bad news, according to James Robertson, whose lab in London was the first to develop a vaccine for an avian form of influenza now in worldwide clinical trials.

"Public health organizations worldwide are keeping a particularly close surveillance on the virus," Robertson said after keynoting the annual conference for state public health workers. "Its herald wave came in an offseason, has a low fatality rate and could well reassort and reappear in late fall, early winter. This is noteworthy because that was exactly the same course the Spanish flu followed in 1918."

That pandemic occurred in the second wave following the initial outbreak the previous spring and summer, he said. And most of the 50 million deaths worldwide that winter were actually caused by a bacterial infection, not the virus itself.

"The virus broke down the immune system in way that bacteria were able to rally in the respiratory system and cause extreme illnesses and death," Robertson said. "The minutia behind what caused it were of no matter; people were dying."

Close scrutiny of the behavior of this new iteration of the virus matters deeply right now during what Robertson called "effectively the staging area."

"Being ubiquitous and abiding gives us a kind of template for flu," he said, "but this strain so far has remained inscrutable — and remember we're only a few weeks into this — but there are signs that it has properties that make it something else entirely, something rare or something new."

The troubling part is the virus seems to lack proteins that are possessed by some strains that would allow it to infect cells outside the body's respiratory system.

"That's what makes this outbreak so exceptionally interesting scientifically and so exceptionally worrisome for public health authorities around the world," Robertson said.

If it is different, it will in effect be marshaling momentum between now and winter, he said, and its wake when it flairs up again could be quite severe.

Biologists manage to stay out front of seasonal flu viruses, which are caused by a host of strains that have come but are never gone. Seasonal vaccines have helped limit flu deaths, mostly among the elderly.

"The problem is that although we're a step ahead, we're always a year behind," Robertson said. "That means that this year's new vaccines are based on what the flu did the previous season and what virologists believe how the virus is likely going to mutate in the meantime."