If there is one thing we should be greatly thankful for, then it should be our immune system. We deal with viruses and nasty bacteria on a daily basis, and without any form of protection, we would have died or probably gone instinct long time ago.
Each one of us has an effective internal defense mechanism in our bodies that prevent harmful bacteria from hitting us. Basically, there are special cells that send warning signals when they spot a virus, so the rest of our system works on preventing it from spreading. It is a very smart system and because of it, we are healthy and strong today.
It is very important to take care of your immune system and to work on making it stronger. Remember that it is your shield from all diseases, so you should be its own helper, especially during the cold season or when you have to be in contact with many strangers.