The influenza virus, ever-wiley, has decided to hit America early and hard this year.  

The flu season is always full of uncertainty.  How bad will it be?  How well will the annual vaccine work?  What folk remedy will the internets come up with this time?

 For many people, all this uncertainty can make a flu shot not seem worth the effort.  That wouldn't be so bad if flu shots were just about our own personal cost-benefit analysis.  

But the truth, which gets far too little attention outside epidemiology circles, is that getting your flu shot is about more than just you.

According to Google flu trends, the flu season has passed "intense" and gone straight to "red arrowhead" levels.

Flu shots are not magic.  Even if virologists correctly guess what strains to include in the vaccine, the jab won't work 100% of the time.  The shot doesn't directly fight off influenza, it trains your immune system what the virus looks like so your white blood cells will be ready for it.

 If your immune system is a network of guard dogs, a vaccine is a lock of hair clipped from the virus that you wave in front of your white blood cells' noses saying "Smell that boy?  Don't let it in!"

The flu shot teaches this guy whom not to let past your defenses.

The problem is, the vaccine's effectiveness relies on the strength of your immune system.  People with weaker immune systems - the elderly, babies, those who are already sick - might not be able to fight off the flu even if their cellular guard dogs see it coming.  But these are exactly the people most likely to get seriously ill, or even to be killed by the flu.  So how do we protect them?

The answer is something epidemiologists call "herd immunity".  Flu viruses can't reproduce and spread without vulnerable hosts, and each healthy person who gets a flu shot is one less easy target for the flu.  If the density of easy targets drops below some critical threshold, the flu gets stopped in its tracks and can't become an epidemic.

This is why it's so important to get your flu shot.  Even if you've never gotten the flu before, even if you hate shots and would rather just lie in bed for a week, even if you've gotten sick despite getting a shot in the past (remember, it gives you better odds of fending off the flu, but doesn't guarantee it).  The flu vaccine is least effective for your grandparents, for your cousin's new baby, and for your neighbor just out of the hospital.  Their best protection, then, is for you to get your shot and fight away the flu before it can ever get to them.