Cotard’s syndrome (CS) — or cotard delusion — is a rare mental illness in which the affected individual holds the brief that they are already dead, do not exist, or are putrefying. The patient may also have the delusional belief that they have lost their blood or internal organs. First documented in the 1800s, the disorder is still a great mystery today. Why? Because the underlying cause isn’t quite yet understood. Read on below to find out more.