 In the 1970s and '80s, evidence suggesting that opioid drug addicts show increased risk of infection (such as increased pneumonia, tuberculosis, and HIV/AIDS) 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.The first step of determining that morphine may affect the immune system was to establish that the opiate receptors known to be expressed on cells of the central nervous system are also expressed on cells of the immune system.