You most likely aren't doing anything wrong... if you have immune issues and are on any steroid treatments for them then your system is basically a 'sitting duck' for any germs that happen along. If you are prone to lung infections and are around folks who are sick then I would also suggest wearing a face mask when in close proximity to anyone who is sick.
That is great advice. I had an employee who wore a face mask and latex gloves whenever he surfed the internet - that might be a tad overboard, ya think?