One word. Fear. People are afraid of being sick. "I don't have time for being ill". "I can't stand pain". Let me give you a little secret, you CAN deal with pain. People survive a few broken bones or a couple of concusions (just be careful as to the seriousness of your concusion before you fall asleep. wait at least 8 hours after a hit to the head before closing your eyes and dreaming). A cough doesn't really kill.
Some people have reason to fear. A few of my friends have been close to death because someone who was sick did not keep themselves contained as the doctors ordered. Because of someone's stupidity, more suffered.
Some mothers would call my mom insane for putting me at the age of two with my cousin every day for a week, wishing I would catch the chicken pox. Guess what. I was so healthy with an amazing immune system, I never have had the chicken pox. And that is fine by me. I might catch and be hospitalized because of the chicken pox later on in life. However, I am not at risk for shingles, which can blind patients if it reaches your eyes, because I have never had the chicken pox. There are some people who never catch it.