This commentary on the role of technology in the educational market....
[Stone Soup 11/4/2012 -- used with permission]
....is just as appropriate for knowledge-workers on sick-days.
Your nose may be flowing freely, your voice gone w-a-y past sexy base-voice and into sepulchral, and you really shouldn't be more than 20 feet away from a bathroom... but that's no excuse not to get a couple of productive hours out of the way from the comfort of your home. No whittling down the Netflix queue for you, slacker.
There is some good and bad here. Some illnesses are merely unpleasant but highly contagious, so staying out of the office (and in my case, public transit as well) is more of a public health thing than a "rest will get you back to work faster" thing. Some make the act of getting to work impossible (20 ft bathroom restriction, for example) but don't impact ability to think. For these kinds of illness, being productive on a 'sick' day is entirely possible.
The bad side comes when a workaholic who really should be spending 16 hours sleeping actually spends 9 hours in front of the computer. Exhaustion will make their work product crap, and they'll likely be sick longer due to lack of recuperating sleep. These are the kinds of people that managers really should say, "...and I don't want to see you online," to. I have seen this exact thing happen, and it turned a full-on working-from-home person into someone who only replied to email twice in a day. It works, so those of you with direct-reports, keep this in mind.
But the thing is... it's so easy to let this kind of workaholic just work. They're an adult, goes the thinking, they can make their own decisions. In the era before telecommuting was common, managers had to forcibly evict these people lest they contaminate the office and cause half the office to be out sick a week later. There is even a term for this: Presenteeism.
Sometimes you really do have to tell someone, "It's OK to take a nap, really. We'll both be better for it."