I have read several studies regarding the claimed immunomodulating effects of opiates and the trend is that they affect the immune system in a negative way.
But from swim's experience (chronic opium use) there seems to be no such effect.
In fact I have, according to swim's experience, seasonal flu and/or other infections have hardly ever occured during chronic use.
On the contrary, prior to using opium, swim suffered from the occasional flu or cold.
So that leads to the notion that there must be some anti-viral/microbial systemic properties associated with natural opiates (at least in the case of opium).
There is no data to refer supporting the above claims, but i might look into it further and come back.