It is important to distinguish between influenza and "other" respiratory infections.  Influenza is the only serious respiratory infection that remains in general circulation in North America.  Get it and you may never be the same.  Although it is usually no big deal, I had it when I was 21, and I got pneumonia as a result.  I could bench 260 lbs before the illness and run miles in under 6 minutes.  I never fully recovered.  I get flu shots every year now.  They are safe and effective, and I sure wish I had had one before that pneumonia.  I thought I was nearly invincible back then though.