When you have something like Parkinson's your body is constantly telling you that something is wrong. I've learned to deal with this by tuning a lot of it out and just powering through. If I paid attention to every muscle twinge, pang of nausea, stomach churn I'd never get anything done.
From time to time though it's not Parkinson's yelling at me to slow up a bit, it's a cold or a tummy bug or tiredness. If I continue to ignore them they normally go away but the fever i have at the moment is a persistent little blighter. Like a runaway train it just takes a little time for the brakes to kick in once I pull the emergency lever. Today it's time to rest. F--- it!