Depends on the experience you have and mindset you're in when you train IMO. Over-training is more of a phase and not a sickness or disease.
When Iick I take it as though I have an even bigger chip on my shoulder. I figure if I can train sick (training is an understatement), then when I recover I will be even mentally stronger. When you're sick your body and metabolism are in overdrive, it's almost a gift to get sick and train, take advantage of it.