"What is Truth?" Pontius Pilate asked, and many others asked, and continue to ask throughout history.
The answers came and continue to come, as varied as there are forms of conviction.
Here is one roundabout response.
If repression and frustration can lead to illness, then so does the repression of truth and the frustrated expression of truth. Thus, when truth is not allowed to come out freely, illness sets in.
Truth is simply that what is natural.