Every year, five companies capture thousands of horseshoe crabs, drain the animals of up to 30% of their blood, and release them back into the wild. It’s the first step in the production of a chemical used to make sure any injection you’ve ever received (from vaccines to pain killers) is free of potentially life-threatening bacteria. At The Atlantic, Alexis Madrigal looks at the impact the harvest has on horseshoe crabs and what might happen to the crabs if and when we come up with a synthetic substitute.