Conventional wisdom long held that the human immune system was no match for cancer. Born of native cells, the logic went, cancer fooled the immune system into concluding it was harmless. Thus protected from attack, cancer easily thrived until its host died.

A deeper understanding of our biological defenses has changed that. The human immune system does battle cancer. But we could better optimize our defenses to fend off malignant disease. That's clear from cancer treatments attempted in New York City and Germany as early as the 19th century. Those experiments and other undervalued evidence from the medical literature suggest that acute infection—in contrast to chronic infection, which sometimes causes cancer—can help a body fight tumors.

It's not the pathogens that do the good work. But the way our bodies respond to the pathogens is key. Infection events, especially those that produce fever, appear to shift the innate human immune system into higher gear. That ultimately improves the performance of crucial biological machinery in the adaptive immune system. This lesson comes, partly, from doctors who risked making patients sicker to try to make them better.