Washington – A flu virus is a powerhouse of evolution, mutating at the maximum speed nature allows. A mild virus can morph into a killer and vice versa. One change already made this year’s swine flu more of a problem, helping it spread more easily among people. The big question is: What mutations are next? That’s why scientists are watching it so closely. “There are no rules to flu viruses; they are just so mutable,” said Dr. Paul Glezen, a flu epidemiologist at the Baylor College of Medicine in Houston. “The fact that it changes all the time really confounds our efforts to control it.”

Think of flu’s evolution like a family tree: In the current flu’s distant ancestry are last century’s three pandemics. But its more immediate relatives are swine flu strains that were no big deal to humans. The good news right now is that this flu has lost some of the most dangerous genetic traits of past pandemics. The bad news is that it’s gained something its parents didn’t have: the ability to spread from human to human.

Flu reproduces about every eight hours, said Dr. Raul Rabadan, professor of computational biology at Columbia University. That means this morning’s flu is a parent by the afternoon, a grandparent by the evening, and a great-grandparent by the next day.

Instead of complex double-helix DNA – nature’s basic biological instruction book – flu has a simpler, single strand of genetic code. Normal DNA has a spellcheck-like system that reduces mistakes in replicating the code; the flu virus does not. So mutations come more often. If the mutations are good for the virus, they multiply, and voila, you have a new and sometimes nastier flu.

Scientists are trying to piece together swine flu’s ever-changing genome, its genetic ancestors and the random mutations that in this instance turned a simple pig disease into something that scares billions. They also don’t know how the virus is going to mutate next.