I've never understood the whole "doctor's note" thing. When you're sick, the last thing you want to do is go out in the cold or rain, ride the train with other possibly-sick people, sit in a doctor's waiting room with other definitely-sick people, just to get a note that says something you might be able to prove with an e-mailed photo of the thermometer you just took your temperature with.
>Who comes up with this shit.
Moving from a small (10 person) company to a building with 1000 people in it I went from almost never being sick to getting sick 3+ times every winter.
Strongly encourage your employees to stay home when they're sick as long as they're getting their jobs done. If they're out frequently or for a long period of time (4+ days) require a doctor's note. That's your sick policy. Done.