If you have been sick a lot, you should be talking to your doctor about why you are getting sick and what you should do about it. This may have nothing to do with your training but it could be that the training has been weakening your immune system and making it harder for your body to deal with whatever is making you sick.

At this point you're probably better off just resting, drinking plenty of fluids, eating a healthy diet and just generally trying to get as healthy as possible. I'm actually surprised your group is doing an 80 mile ride two weeks before the century - I would think you'd be tapering at this point. The training you've been doing all summer is what matters most, and now you need to rest to make sure you're not worn out for the big ride.