But the truth is that nobody knows what causes autism or what exactly accounts for the recent rise in cases.

但实际上，没有人知道自闭症的真正起因，也没有人知道近年来此类病例上升的真正原因。

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There are a lot of theories -- nobody really knows what causes it.

CNN: Smoking quadruples risk for vision-stealing eye disease

Nobody knows exactly what causes the signs and symptoms of post-polio syndrome to appear so many years after the first episode of polio.

CNN: Post-polio syndrome