15 -20 days in a year is normal. everyone get sick and has the normal slump in productivity. tell them to take it easy on me for a day or two.
if there are important things to take care off at your work or somewhere. here are my advises.
1) I think that the sickness is more about your state of mind . The more you lay on your couch feeling bad, the more you will feel sick . Force yourself out of the couch and go and face the world. I assure you will start feeling good straightaway.
2) Having a positive self talk will definitely help on those days. Try to make a step by step goals. This will help you to get motivated and productive.
3) I started feeling better as soon i finish 15- 20 minutes jumping , stretching etc on my low feeling sick days.
4) Get a hot shower before the start of the day.
5) Overall having a exercise and some jogging routine 365 days a year will bring feeling not well days very much down. Many including myself has underestimated it's power but when started doing it regularly, it done wonders for me on every front in my life.
6) Make your important task interesting. This is about overall attitude how you live your life. The more positive you are in your day to day life will bring more positiveness even if you are sick.
7) Cheer up your mood, this is life and everyone has it's share of ups and down.