For a month, writer Esmé Weijun Wang was fully convinced that she was dead.

In an essay about this experience, Wang shared how she felt like she was just aimlessly walking on the face of the earth.

"I was doomed to wander forever in a world that was not mine, in a body that was not mine; I was doomed to be surrounded by creatures and so-called people that mimicked the lovely world that I'd once known, but... could evoke no emotion in me," she said, as quoted by The Washington Post.

This episode began after Wang fell unconscious for four hours in a flight home from the United States to London. She thought she died on that flight.

"I was convinced that I had died on that flight, and I was in the afterlife and hadn't realized it until that moment. That was the beginning of when I was convinced that I was dead. But I wasn't upset about it, because I thought that I could do things [in my life] over and do them better," she said.

She also recalled feeling scatterbrained and losing grip of reality weeks before she began feeling she was dead.

Wang's perception that she died suddenly ended one morning when she woke up beside her husband.

What Wang experienced was a rare mental illness called Cotard disease, which causes a patient to feel like he or she does not exist.

Psychiatrist Jesús Ramírez-Bermúdez at the National Institute of Neurology and Neurosurgery of Mexico, who specialises in this illness, said it can be treated through psychotherapy and medication.

He added that patients suffering from this illness, which has been also called the "walking source syndrome," suffer delusions, which may cause them to have suicidal tendencies.

"Patients truly experience all kinds of suffering," the psychiatrist shared with The Washington Post.