In a landmark 1975 study, Ader and Cohen demonstrated conditioned immunosuppression in rats. They paired flavored water (a conditioned stimulus) with an immunosuppressive drug (an unconditioned stimulus) that naturally caused sickness. After this conditioning period, the researchers found that the flavored water alone could trigger immunosuppression (a conditioned response). This experiment provided clear, direct evidence that the immune system's functioning could be altered through classical conditioning.