Good question, I've been sick only once while building this habit, and it surely depends on what type of sick you are. I had a sore throat, and drinking a glass of water immediately helped me feel more energized and able to do light exercise. If you ran for 30 minutes the previous day, and now you're sick, change up the exercise. Do a little dancing to your favorite playlist, or some jump rope. Whatever you're able to do is far better for building your habits than doing nothing at all.