[Figured I'd finish writing an older post.]
Historically, infectious agents are ultimately limited by their own success; a virus which is too successful kills its host and isolates itself from further propagation mechanisms. A virus which isn't successful enough doesn't spread rapidly enough to propagate itself to begin with. There's a fairly narrow window between these two failure modes wherein success lays.
Even if medicine were to make headway against infectious agents, it would only serve to move these windows over. Medicine gets added to the arms race, rather than eliminating it. The excesses of an infectious agent no longer lead to the destruction of its own viability; modern medicine helps maintain its hosts and return them to the pool it depends upon.
The campaign against infectious agents therefore must be all or nothing; the present strategy of amelioration is ultimately unsustainable.
Of course, it's possible we're merely delaying a "proper" solution until the day we're able to complete it. The question that remains is whether our delays make a proper solution impossible, or at least delay its implementation as well; had we had the antibiotics a hundred years ago that we possess today, would broad-spectrum treatments have been survivable for any specimen?