It depends on how sick, what kind of sick, and the weather. I think of it this way:
1. If I'm having IBS issues, I stay indoors with Cathe. Often, the workout will help alleviate my symptoms. Endorphins
are a pain killer

.
2. If I have a cold and I'm not having a fever, I just go anyway.
3. If I have a fever, no exercise at all.
4. If I have a headache, then I do the yoga. Increasing my blood pressure can make that sort of thing worse.
5. If I have a broken bone, sprain, or strain on an arm, I will do my normal workouts just no strength training, and no yoga (downward dog is out of the question at that point.) If a knee is involved, then definitely, no yoga.
I think of it this way, if I'm actually sick, then I need to let my body take care of itself. Thats not the time to add more stress. Exercise is a way to alleviate emotional stress, but bare in mind, that it does stress the body. In the long run, there is no real problem in having a few days off. I just took a day off last week because I felt lousy, and just did yoga. I wasn't classically sick, but, I didn't feel like it. Most of the time, I wake up raring to go, no problem so when I do feel like that, I take the rest. In the whole scheme of things, one day or the other in terms of my weight loss or my fitness won't make a difference. Its the general picture that matters.
The question that I ask myself is, "Will I feel worse or better if I workout?" Most of the time, the answer is "Lets go workout, I know I will feel better."