Morphine has long been known

Morphine has long been known to act on receptors expressed on cells of the central nervous system resulting in pain relief and analgesia. In the 1970s and ’80s, evidence suggesting that opiate drug addicts show increased risk of infection (such as increased pneumonia, tuberculosis, and HIV) led scientists to believe that morphine may also affect the immune system. This possibility increased interest in the effect of chronic morphine use on the immune system.