Ever wonder why your body freaks out over shrimp or pollen like it’s preparing for the apocalypse? Well, turns out your immune system is still stuck in the past, fighting worms. Yep, in the good old days (pre-hygiene, pre-soap, pre-toilet-paper-separation), our ancestors had to battle worms daily. Now that we’ve mostly gotten rid of those little critters, our immune systems have absolutely no idea what to do with themselves—so they just go off on anything that looks vaguely suspicious. Watch Kurzgesagt’s video to learn why your body is basically throwing a tantrum every time you eat shellfish.