I would think, though, that if you are always medicating and premedicating for everything as some do, the genetics of immune systems break down. For instance, one organic method for disease control (the one I use) is to cull if the animal is unable to fight off a disease on its own. This way, the only critters you ever have in your flock are the ones that have the genetics for strong immune systems. By medicating for everything, the animal is given a crutch and those with naturally weak immune systems survive to pass on their traits.
So, in a sense, it is true that you create both stronger bugs, and weaker hosts that need the medicinal crutch to survive. A double whammy.
Least, that makes sense to me, anyway.