From the article:
"In 1975, a psychologist in New York was studying taste aversion in a group of rats and got an utterly mystifying result.
"Robert Ader, working at the University of Rochester, gave his animals saccharin solution to drink. Rats usually love the sweet taste but for this experiment, Ader paired the drink with injections of Cytoxan, which made them feel sick. When he later gave the animals the sweetened water on its own they refused to drink it, just as he expected. So to find out how long the learned aversion would last, he force-fed this harmless drink to them using an eyedropper. But the rats didn’t forget. Instead, one by one, they died.
"Though Cytoxan is toxic, Ader’s rats hadn’t received anything close to a fatal dose. Instead, after a series of other experiments, Ader concluded that when the animals received saccharin and the drug together, they hadn’t just associated the sweet taste with feeling sick, they’d also learned the immunosuppression. Eventually, they’d responded to the sweetened water just as they had to the drug. Even though the second phase of the experiment involved no drug at all, the doses of water Ader fed them suppressed their immune systems so dramatically that they succumbed to fatal infections."