staying healthy

Hi everyone,
I'm looking for some advice or tips. I am new to the role of "office nurse". I worked in a hospital for 10 years and then for an agency for 3.
Since transfering to a family practice clinic I have found that when patients come in with colds or the flu I inevitably come down with a bug too and lose time from work. I've come to realise how many people in this world don't cover their mouths when they sneeze or cough.Good ole airborne germs!
The only time I got sick like this was when I was a new grad and had just started working.
My guess is I need to rebuild my immunity.
We don't have a sink handy in the front office and when it's busy you can't always leave to wash your hands.
I get the flu shot every year, I am always careful not to touch my face, I wash my hands before eating, doing tasks etc., I'm even cleaning off our phones with alcohol swabs after patients use them and when I start my shift. (Am I sounding paranoid?:-)
Anyway has anyone else been through this? Will it only take time to toughen up my resistance? Please, I welcome any suggestions or comments.
Thanks, Lisi