It is neither contagious nor genetic, and no one knows what causes the permanent variety.

它既不传染也不遗传，没有人知道什么导致了它不停地变化。

youdao

Like "classic" or "typical" asthma, no one really knows what causes cough-variant asthma.

就像“典型”哮喘，没有人真正知道究竟是什么原因造成了咳嗽变异性哮喘。

youdao

Nishibayashi said, however, that no one knows what causes autism.

CNN: Opting out of vaccinations could get tougher in California

In fact, no one knows what causes these symptoms in most babies, and in most of these cases, it seems that doctors are essentially administering a placebo to treat the anxieties of the parents, and doing little to help the child and in theory exposing them to a very slight risk of harm, although these are generally thought to be exceedingly safe medications.

FORBES: Are doctors over-responsive to patients -- or not attentive enough? (Answer: yes.)