The precise nature of his illness has not been made public.

The exact nature of her illness has not been disclosed.

However, the nature and severity of her illness were never revealed.

She declined to elaborate on the nature of his illness.

We do not know anything about the nature of her illness.

But the nature of his illness has made this season much more challenging.

She said it like I should have known the nature of his illness.

More recently, there has been much speculation as to the nature of his illness.

Did Tony say anything at all about the nature of her illness?

There was no comment on the nature of his illness and he did not immediately seek treatment.