In sickness and in health - should I run through the pain?

Anyone that has trained for a marathon will have inevitably endured sickness at some stage of the training programme and reached that dilemna - Should I rest and get healthy or follow the training programme as laid out for me? But what is the right thing to do?
I have found in my experience it really depends on the kind of illness that has gripped you. A simple head cold might not stop that easy run or light training day whereas an all out stomach bug puts paid to all type of training.
As a general rule at times, my own thinking is, sickness above the shoulders (head cold, sinuses etc) may still allow light training (I still wouldn't attempt the tempo or long run with any sickness) and below the shoulders (chest infection, stomach bug etc) I would just rest and recuperate.
It can be very hard to discipline yourself to take that rest (especially if training is going well) but the long term repercussions may be a lot worse and a few days of bed rest (or at least a break from training) is easy to get over and is a lot better than having that illness drag on resulting in more time in sick bay and maybe having to forego that marathon you have been training so hard for altogether - it's the bigger picture that counts!