This made me chuckle. Depends on whose opinion you favor, the ‘various’ crowd or your own. Unless, of course, you’ve done extensive testing to support yours.
Depends, I suppose, on what is meant by disease. Every tank contains a large variety of bacteria which can opportunistically infect fish. If you mean something like ich, then I suspect more tanks have it than not, particularly if you’ve not used any kind of QT. It is not unusual for a tank that is asymptomatic to suddenly display ich upon the introduction of a canary.