This year marks the 100th anniversary of what's called the "greatest pandemic in history. -- the 1918 flu virus, also called the Spanish flu. That flu lead to the deaths of between 50 and 100 million people. Many people may think that an outbreak of that magnitude couldn't happen in today's world, but doctors say we are still vulnerable to pandemics. Our guests discuss when the next worldwide pandemic will strike, what kind of flu strains we might see, and the latest research on flu viruses. In