Evolutionary biology may sometimes seem like an arcane academic pursuit, but just try telling that to Gavin Smith, a virologist at Hong Kong University. For the past week, Dr. Smith and six other experts on influenza in Hong Kong, Arizona, California and Britain have been furiously analyzing the new swine flu to figure out how and when it evolved. The first viruses from the outbreak were isolated late last month, but Dr. Smith and his colleagues report on their Web site that the most recent common ancestor of the new viruses existed 6 to 11 months ago. “It could just have been going under the radar,” Dr. Smith said.

The current outbreak shows how complex and mysterious the evolution of viruses is. That complexity and mystery are all the more remarkable because a virus is life reduced to its essentials. A human influenza virus, for example, is a protein shell measuring about five-millionths of an inch across, with 10 genes inside. (We have about 20,000.) Some viruses use DNA, like we do, to encode their genes. Others, like the influenza virus, use single-strand RNA. But viruses all have one thing in common, said Roland Wolkowicz, a molecular virologist at San Diego State University: they all reproduce by disintegrating and then reforming.

A human flu virus, for example, latches onto a cell in the lining of the nose or throat. It manipulates a receptor on the cell so that the cell engulfs it, whereupon the virus’s genes are released from its protein shell. The host cell begins making genes and proteins that spontaneously assemble into new viruses. “No other entity out there is able to do that,” Dr. Wolkowicz said. “To me, this is what defines a virus.”