Scientists around the world are pursuing the tiny spheres — each one half the size of the smallest bacteria and laden with 500 disease-bearing spikes — that are the viral couriers of the swine flu outbreak.

It has a comparatively simple genetic structure, with eight strands of RNA genetic material totaling only about 14,000 letters of genetic code in all. By comparison, the fruit fly has 165 million letters in its genetic code.

These self-replicating biological devices invade cells, hijack their machinery to make more invaders, then burst free to repeat the cycle. "They are incredibly elegant little machines," said virologist Vincent Racaniello of Columbia University.

In April, early details of the outbreak alarmed health officials. The disease belonged to the H1N1 family of flu that struck in the 1918 flu pandemic that killed as many as 100 million people worldwide. And if it were a genuine pig virus, that would mean people would have little immunity built up from exposure to related influenza.

"Right now, everyone is clamoring for empirical data on the natural history of the new strain," said epidemiologist Derek Cummings of Johns Hopkins University.

On Friday, researchers at Columbia University published an analysis confirming that the virus was truly a swine flu, with most of its genetic material derived from an infection seen in North American pigs and the rest from Asian ones.

The data reassured physicians that the swine flu lacks a gene linked to the deadliness of the 1918 flu. On the downside, analysis showed the swine flu virus shares only about 80 percent of its genes with the human H1N1 virus in vaccinations last year, which limits the immunity that people may have.

Influenza can quickly re-sort gene segments when people or pigs are simultaneously infected with two or more strains. Inside infected cells, virus proteins are produced willy-nilly as the bug reproduces and are then reassembled haphazardly, allowing for the shuffling of RNA segments.

The swine flu could change significantly in coming months, making treatments less effective.

The 1918 epidemic simmered throughout the summer and then burst out among the public in the fall.