The body of a dead crab produces histamine, which is one of the allergy-inducing substances, so consuming half-dead crabs is more likely to induce rashes, itching, skin redness and swelling, and even respiratory difficulties, anaphylactic shock and other allergic symptoms compared to fresh crabs.

Crab is a scavenging animal, and many bacteria and other microorganisms remain in the body, which can resist microbial attack when the crab is alive. But when the crab is dying, the protein activity in the body decreases, and microorganisms will rapidly grow.