More than 80 years ago, manufacturers started spiking vaccines with alum, an additive, termed an adjuvant, that spurs a stronger reaction from the immune system. Yet scientists have struggled to explain exactly how alum, a catch-all term for several types of aluminum-containing adjuvants, does this. Recently, researchers have floated at least three possible mechanisms, including one that involves DNA spilled from dying cells. The reason alum works so well, several studies suggest, is that it trips an alarm that alerts the immune system when cells are in trouble. Insights into how alum works might allow researchers to design replacements that retain alum's advantages but lose some of its shortcomings.