Sunny is not sick. She is over the idol lifestyle and has been for awhile now. Read her interview from awhile back where she talks about how she just wants privacy and a normal life. Before that she talked about how she regretted becoming an idol at such a young age. This is basically the same thing that happened to Sulli. You can lie to yourself and say she's sick if it makes you feel better but don't be surprised when Sunny is the next member to leave SNSD.