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This question originally appeared on Quora. Answer by David Chan, MD.

The large majority of viral infections don’t cause symptoms because young healthy adults have immune systems that suppress them. The infections may cause only a few days of fatigue in it’s mildest form. They get what are known as subclinical infections. They are infected, not all that ill, and pass it along to others with a handshake, a hug, a cough or a sneeze.

However some people aren’t as fortunate. They may be immunosuppressed because they are being treated for leukemia or other cancers, they may be organ transplant recipients or they may simply be old.

In a bad flu season in America, 30,000 people die from a flu complication. That is equal to the number of Americans killed in auto accidents.

That you haven’t yet been killed in an auto accident doesn’t mean that you shouldn’t wear a seatbelt, maintain your car, be attentive with driving and obey the driving laws. But there is another aspect to driving also. You might not kill yourself but you could run over a curb and kill 5 kids waiting in line for their schoolbus.

So too with vaccines. If you’ve not gotten MMR, plenty of other people have had theirs (at least until recently) so there is herd immunity. But when that no longer is the case, you’re putting yourself at risk for some serious complications. Adults tend to be much more ill than children with this stuff.

But let’s remove your own health out of the equation. Think of getting a flu vaccine as equivalent to being a careful driver. You might not feel that bad getting the flu but you’re protecting that first grader on chemotherapy for acute lympoblastic leukemia or that 25 year old behind the counter at 7 Eleven who 3 years ago got a kidney transplant from her brother, or that 60-year-old man with emphysema waiting for the birth of his grandson. These people can and do die from a severe bout of the flu.

Step up and do the right thing.

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