mmm yes, one cannot be totally immune to the ever changing world of microbes.
consider the stress response. sympathetic stimulation via exogenous methods such as amphetamine or the 'psycho-endogenous' would cause a marked increase in adrenocorticotropic hormone and thus cortisol. this is going to suppress the immune system as the body is geared up for the 'here & now' IE fighting off a sabre tooth. when relaxed, these hormones are less abundant (circulation). the body intricately produces its leukocytes and they probably live a lot longer in this mode.
the opiates will decrease the stress response/hormones significantly, the above process may exceed what is normally expected of it. one would not be newly immune to anything in particular, just have a marked increase in WBC count in general. higher WBC count would make an infection/invasion more prone to quick phagocytosis. thus the infection does not last as long and may not even be noticeable, but it was there.
this is based on the assumption that what was read was true. that is, opiates suppress adrenocorticotropic hormone. some opiates do, some dont. some will likely have a mix of both inhibition and excitation. it is up to the individual to do the research.
ive been wanting to put up a collection of information regarding this for a while but have not had the time. this would be a good thread to start posting any info you have regarding opiates and their direct effect on ACTH, cortisol, and other glucocorticoids.