Being sick - a cold, flu or sore throat - is never planned or welcome in a triathletes life. The general rule is if the symptoms are above the neck you can train through it with low intensity exercise and below the neck (sore throat etc.) complete rest is best.
Sometimes it's just unavoidable luck that you've picked up an infection. You might also interpret the sickness as your body giving you a hint. Often, looking back and being honest with yourself, you'll see that you've weakened your immunity by skipping sleep, over training or eating rubbish.
Not being able to train as planned will drive almost any triathlete mad. To mentally survive consider the much needed rest your muscles are getting. On regular training "rest days" most triathletes are working, doing the groceries and running around after the kids. When you're sick you're in bed, horizontal and your muscles are REALLY recovering.
It's also a chance to surf the web reassess your training, check out race websites and get excited about the season. Just don't rev yourself up so much you try and go back before you're ready. Enjoy the rest and I suggest waiting 12-24 hrs after I feel I COULD train before actually getting out there.