Numbers 14:18 King James Version (KJV)
18 The Lord is longsuffering, and of great mercy, forgiving iniquity and transgression, and by no means clearing the guilty, visiting the iniquity of the fathers upon the children unto the third and fourth generation.
The Sins of the Fathers is visited upon the children! The vilest of Sins committed are often hidden from other men and cannot be hidden from Elohim/God, as Elohim/God sees all things. Mental and Physical Diseases are the product of Sin.