The sight of pink residue creeping around drains and lurking in grout lines is enough to make anyone feel uneasy about their bathroom’s cleanliness. Recently, discussions about this mysterious pink substance have been popping up online, and it’s fascinating how many people share this common household nemesis.

Let’s get scientific for a moment. This pink stuff isn’t actually mold, despite what many believe. It’s a bacteria called Serratia marcescens, and it has quite an interesting history. The most intriguing part? During the Cold War, the US military actually used it in biowarfare testing because they thought it was harmless. Spoiler alert: they were wrong about that last part.