The explanation for the fever producing effect is on much stronger ground. Studies have shown that the immune system works more efficiently at higher temperatures— specifically the proliferation of the fighter cells is accelerated at higher temperatures. A wide variety of viruses and bacteria thrive at temperatures below 98.6°. As the body temperature rises, their doubling time slows and in some cases stops altogether.
So, it makes sense to have a fever when the body is fighting an infection. In some animal experiments, scientists have found that blocking a fever in an infected animal makes it less likely to survive the infection. These studies need to be corroborated by more research but if proven it will be a surprising twist to the accepted wisdom of treating the fever with medicines like aspirin.
One other point of interest is the use of these interleukins in the treatment of various diseases like cancer. They cause the body’s immune system to be activated but they have the nasty side effect of making you feel stupendously sick—something reported by cancer patients treated with interleukin-2.