I second morpiko ( actually this is my fav mosquito bite cream ) thesis. Germs and bad bacs are everywhere, in the water,in the air and inthe fish inside and outside. If the fishes are healthy, they will not be infected . Once they are weakened, for example due to bad water quality,all the bad virus will stake their claims.
I am also against fish mediciation. You probably kill the fish faster.
To each its own, no right or wrong.
cheers