Talk:Muzan Kibutsuji/@comment-39803894-20200212012018/@comment-35873778-20200212013259

From the history section under the synopsis tab:

"As a young human man, Muzan was diagnosed with a disease that would kill him before he reached the age of 20. The doctor treating Muzan gave him a special prototype medicine. In a fit of rage, resulting from his worsening condition, Muzan killed the doctor. However, only after killing the doctor did he realize that the medicine was actually working. He had acquired a strong body. Muzan had no problems adjusting to having to kill other humans for survival, but the problem he faced was the fact that he was unable to walk in the sun."

Basically, the special medicine he received as 'treatment' for his disease turned him into a Demon.