Train. You will feel better. Take a bunch of caffeine, l-arginine some beta alinine. If I am ever sick, which is a rarity, I look forward to going to the gym because I know for the 1-2.5 hours I am there I will be essentially symptom free. Any congestion I have tends to dissipate, the movement raises my body temperature and I have broken a couple of fevers that way. Then I am able to go home and fall asleep relatively easy.
Obviously if you are at deaths door avoid this and get rest. It is a judgement call.
Shelby Starnes said something that really inspired me. It was during one of his interviews and someone asked him a question on his motivation or something like that. His response was that when he is not feeling motivated or feeling like taking a day off or a cheat meal he thinks wwowd. Which is a play on wwjd, what would jeses do. In this case wwowd stands for what would the over all winner do. I don't know how into bodybuilding you are or what your goals are but I found that inspiring and it helps me in my moments of weakness.