Mangetout makes some good points: To put things bluntly, a virus is a bit of genetic material (DNA or RNA) wrapped in a protein shell. The protein can be relatively complex, but it’s nothing on the order of a living cell. It isn’t alive, which means it cannot metabolize food, react to stimuli, or self-repair. It also lacks a cellular structure and cannot reproduce without a host, but it can hijack a cell to reproduce copies of itself.
So, since it’s so simple, why can’t we develop drugs that simply rip the protien to shreds? Well, something that blunt would probably do a number on the host’s proteins. Reducing your patient to a slurry of simple carbon compounds isn’t a way to get repeat business. Why can’t we develop our own proteins that interfere with the virus’s reproductive mechanisms? Well, I think Interferon does just that. But it’s expensive, not completely effective, and we’re faced with a moving target: Viruses mutate at a rate only matched by similar vermin, such as spammers and computer crackers. To develop a drug that works against this year’s mix of flu viruses will do you next to no good against next year’s, and it will be powerless against the latest superstrain to come out of Asia.
That is why getting a flu shot every year is important: A flu shot isn’t a drug, but a mix of proteins culled from inactive (That which is dead cannot die.) viruses from that year’s mix of strains. Your immune system, effective machine it is (in most people), gets mug shots' of the proteins (in the form of T-cells) and puts out an APB’ for anything that looks like them (in the form of those T-cells glomming on to them and making them more visible to your white cells, which proceed to eat the whole package).
In this case, close enough can count: If your T-cells get 70% of the viruses in your body, you’ll feel much better for it (maybe you’ll get off with a slight runny nose and some minor fatigue). Which is why flu shots can frequently reduce the intensity and duration of the common cold: Apparently, the mix of flu viruses has something in common with the mix of cold virurses.
A magic bullet would be very nice and would make a lot of people very rich, but I just don’t see it coming out of current methods. The drug would have to be both highly specific and highly nonspecific, to both leave your own cells alone and to get all possible variations on the given virus. And curing one virus would not make it trivial to cure another, but it might be a crucial first dozen steps.
So, why are bacteria a solved problem? Well, they’re not. Certain strains of TB (tuberculosis), for example, have become all but immune to everything we have. Tetracycline, a very expensive and rarely-used drug, is the only weapon we have left against that scourge. Other infections have become superbugs, as they’re called, and are immune to 99% of all traditional antibiotics. So, how do antibiotics work and how do bugs become superbugs?
Basic antibiotics
Antibiotics hijack the cell’s reproductive machinery. They clog up the works so the cell can’t reproduce, and let the cell die on its own. It’s a miracle, in that it doesn’t affect us nearly as much (though stronger antibiotics can cause problems of their own) but ends infections in a rather clean way. Of course, their effectiveness is dependent upon how, exactly, the cells reproduce. Mutation can change that.
Bug and Superbug
To misquote Friedreich Nietzsche, Superbugs have “The Will to Mutate”: They’ve mutated into something most antibiotics aren’t effective against. Their reproductive machinery is so different from the average that the usual mainstays (penicillin, amoxicillin, etc.) won’t work. We need the heavy artillery to kill them, and we hope against hope that those drugs work. I’ve never heard of Tetracycline-resistant TB, but it’s certainly possible.
"Apres moi, le deluge"
–King Louis XIV, noted antibiotic. 
So, how do superbugs get created? Misuse of antibiotics. Every time someone takes less than they should, or uses them as a remedy for something that isn’t bacterial, they kill the weak and leave the strong standing. The strong reproduce, and they are that much more resistant to that antibiotic in the future. If a member of a slightly resistant strain gets hit with a different drug that doesn’t wipe out its descendants, those offspring will have resistance to two forms of antibiotics. Don’t rinse, and repeat. Eventually, you end up with people home-growing their own bioweapons. There’s a lesson here: Use antibiotics wisely. Don’t take them as placebos, and don’t take them until you feel better.
