unexplained

A death whose cause is not immediately known.

A situation where someone has vanished without a known reason.

不明原因的疾病

An illness for which doctors cannot find a cause.

医生找不到病因的疾病。

An absence for which no reason is known.

未解之谜

A mystery that has not been solved or understood.

尚未被解决或理解的谜团。