I remembered this thread and thought about it recently. I've been doing back-to-back tattoo sessions the past month or so, and I can say it really wacks your immune system hard. That or I have really weird timing. I ended up getting the flu one day after a big back-to-back tattoo weekend, a few weeks ago. I just had a pretty brutal tattoo weekend this weekend, and I ended up getting a pretty bad cold all week. I can't say for sure they are related but the obvious thing is, if you are sick and trying to heal a big tattoo, it can slow down the process drastically.