The 1976 convention of the American Legion in Philadelphia turned out to be an ill-fated event: Around 200 attendees contracted pneumonia, and about 30 eventually succumbed to the disease, which was caused by a hitherto unknown but quickly famed bacterium: Legionella pneumophila. This microbe evades the immune response by a particularly sneaky mechanism. It lets itself be eaten by the host’s macrophages, and instead of perishing, like regular pathogens, it makes itself a cozy home within those immune cells. It does this by preventing the fusion of the phagosome, where the bacteria reside, with the lysosome, which carries the killer enzymes that usually tackle the invading microbes, inside the macrophages. Instead, the bacteria continue to replicate happily inside the immune cells until these burst, releasing thousands of new bacteria that can hunt for new macrophages, thereby spreading all over the body. The disease is often deadly, especially when antibiotic treatment is delayed.