This can't be more obvious, so Head doctor is Tooru stand, we know its main ability is the calamity pursue, and when you are affected by it you can see the doctor like if he is pursuing you, then we can see is a super powerful stand, can affect multiple people at the same time so this could be the reason why non stand users can see the doctor too and for a better disguise it looks and acts like a 89 year old man. Right? + i really hope a Joshu redemption in the next chapter and he apologizes for everything he did to Yasuho, honestly could be really epic to see Joshu but without being an asshole.