And pandemics are so common these days. Flu, if you get it you get over it, unless you have other medical problems (the ones the scientist should be working on in my opinion - saying that they probably are but coming up with jack).
One could argue that the lack of recent pandemics is because
of vaccination programs (combined with a bit of luck and sensible disease management).
No vaccinations == more people infected == more chance for dangerous mutations to occur.
Interestingly, Spanish Flu was most dangerous to young adults.