If you're sick, stay home. Most people don't go to a doctor but treat their temporary cold/flu/upset stomach at home. Running out to get a doctor's permission slip is counterproductive. Why sit two hours with other sick people in a doctors office waiting for him to tell you you're "sick" and there is nothing much he can do about it?
I certainly never want either myself or other people at work in a sick condition. Not only do they perform poorly, but they infect other, healthy people. It drives me crazy when somebody who is obviously sick shows up and infects everybody else.
A good solution is to make a courtesy phone call sometime during the day to check up on the employee's health and see when they think they will get back to work so you can make plans to cover for them if they think they will be out for a while. That should prevent a lot of abuse.
You can't do that, according to unions. The company I work for recently had managers begin calling the missing employee, and if there was no answer, they dispatched someone to the employees home to do a 'welfare check'. The union demanded they cease and desist or in addtion to filing grievances, they would file civil rights lawsuits and go to the press.