The average American household gives "birth" to two children.
And approximately every 20 or so years, those children undergo tiny mutations that make them stronger, more adaptable even, to the environment they're living in.
Compare that to the flu virus, which gives birth to billions and billions of progeny each year and those offspring mutate constantly. Some get stronger, others die off.
It was because of this cycle that the most common virus circulating during this past flu season — an H1N1 influenza A strain that was much different than the current H1N1 swine flu virus now circulating — was resistant to the anti-flu drug Tamiflu.
"The flu virus like most other viruses actually mutates all the time and mutates much faster than cellular organisms and higher life forms," said Dr. Frank Esper, infectious disease specialist with University Hospitals Rainbow Babies & Children's Hospital in Cleveland. "The virus mutates and then the mutation continues on. It really goes by survival of the fittest."
Now take the swine flu, which is a super-mutation of sorts involving the convergence of three different flu strains (swine, human and bird flu) in one organism — in this case it's presumed to have been a pig — and mutating into an H1N1 influenza strain never before seen by scientists.
Although swine flu is currently responsive to Tamiflu, which lessens symptoms of the virus if used within days of the initial infection, health experts worry the virus may mutate and come back next year stronger and possibly resistant to anti-flu drugs.
And just because it didn't turn out to be the big one this time around, doesn't mean world health experts weren't right to be concerned.
"It had the potential to shift into the kind of influenza strain that leads to the kind of pandemic we saw not only in 1918, but also in the 1950s and 1960s, which killed millions of people," he said. "And even if a flu doesn't kill 50 million people, it can still be a very virulent strain. Look at something like [bird flu], that influenza kills 60 percent of the people it infects. Luckily, it hasn't shifted to human-to-human transmission, but it could."
But should you be worried?
"I don't think people should be any more worried [about the flu] than they were last year," Esper said. "Part of living in this world is living with viruses and bacteria that can cause disease. Just like living in this world means living with sharks and polar bears that can take big chunks of skin out of you.
"Science is kind of mutating along with the flu virus, learning how to react to viruses and change how they react to viruses as new ones come along," he continued. "And as we react to the challenges, we'll continue to come up with new methods and therapies to fight these viruses."
The biggest challenge now is coming up with faster ways to make flu vaccines. The current method of using eggs to make a vaccine can take up to six months.
"We usually have to decide what strains will circulate and what they will look like several months before flu season starts," Esper said. "Basically, the flu vaccine is our best guess and sometimes we make a mistake and don't end up with a very good match. What we need is quicker vaccine production using cell cultures versus egg cultures. And scientists have been working on that so I think we will see that down the road."