I have done some further digging, and actually founds some archived similar threads. It appears that 85% of respondents report similar experiences in never actually getting (or at least feeling) sicknesses like the common cold or flu. About 10% reported getting sick once or twice, and 5% stated they got sick all the time. Some common explanations:
1) As Lorne has stated, the theory is that you DO get sick, but that you just dont feel it due to the physical state you have put yourself in, as well as side effects of the drug itself. However, even if this is true, if you can take opiates and not even know you are sick, who cares?
1.2) A hybrid, and very logical ideal includes one post by bben that states, "
opiates are potent immunosuppressants so they may very well prevent the normal response to bacteria/virus we feel as sickness. It prevents elevated cytokines and nitric oxide production in response to sickness among many other actions. This immunosuppression combined with greater analgesia makes it no huge shock that opiate no only block the bodies response to sickness from immunosuppression but also masks it through pain killing and a general mood boost."
2) Other theories involve changed body PH, or changed body temperature.
3) My obscure observation is that it could also align with a common internet "cure for the common cold" that presumes you get most sicknesses via the ear canal instead of more commonly though routs. Since for me opiates seem to affect my inner ear and wax production, maybe something it it...who knows.
In the threads I read it is also interesting to mention that some brought up the fact that morphine was previously sold as a cure to the common cold 100 years ago, and another posted noted that some places with heavy opiate habits (I believe he stated India at the time of writing) had public acceptance that opiate users were virtually immune to colds.
Here is one of the links in case you are interested like I:
http://www.bluelight.org/vb/archive/index.php/t-548596.html
P.S. I am definitely not claiming they make you healthier!!! I am just wondering if they don't create a physical environment that is unfavorable for bacterial and/or viral infections. I do also concede the possibility of symptoms just being masked, though I have a hard time with accepting this. The "statistics" of experience are quite interesting.