If anything opiates are suppressing your immune system, which is why you don't ever feel sick, infections are running rampant but your body is ignoring them. Most of the symptoms of feeling sick are caused by your body fighting the infection, not the infection itself(think fever, runny nose, sputum in your lungs, sore throat from inflammation, vomiting and diarrhea to expel infections from your GI tract, etc. etc.) these are all triggered by your bodies immune system in an attempt to fight the infection, not the infection itself.
dammit