No, this is not me recommending a great work out. For me the easiest way to interrupt any sort of work out schedule is by getting sick. Whether it's allergies, some sort of cold, or flu-like symptoms, it really just takes any desire to exercise right out of me. Even if the motivation is still there, thinking about the complications of a potentially runny nose while running is a real turn off. One of the worst parts of being sick, though, is just how run down you tend to feel, and the last thing you want to do is put any more strain on your body, even if it might just be what you need.
I have, in the past, been so tired of being sick and feeling like I shouldn't work out that I just sucked it up and worked out. I don't know if it was a good idea but, for this instance in particular, I felt great. The endorphin release from working out really helped pick up my mood and make me feel like I was kicking the cold. Not only that but taking a long, hot shower after a workout or a long run can also help you feel, in the case of head colds, like you can breathe easier.
It may not feel like the greatest idea to work out while you are sick, but power through it, find that drive, and you might just feel a bit better. And if you don't feel better, you'll at least know that you didn't skip on your work out and that will feel like a victory in itself.
P.S. Bring some tissues.