Viruses are tricky for a host of reasons: There are many different types, so a drug that fights one may not fight another. They use our cells' own machinery to replicate, so often drugs that would fight them would be toxic to our bodies.
Plus they replicate in huge numbers, and often sloppily -- producing many new forms. If one of those rare new forms happens to be resistant to an anti-viral drug, it will have a selective advantage and multiply -- and pretty soon you have a drug-resistant strain on your hands.
It obviously would be really useful if scientists could come up with a potent antiviral therapy that could be used against a broad array of viruses. Researchers at MIT's Lincoln Laboratory think they're on the way to doing just that. (They published their results in the journal PLOS One.)
The strategy takes advantage of a molecule called double-stranded RNA, which is produced by many, many viruses when they infect mammalian cells. Uninfected, our cells usually don't make this double-stranded RNA, and to some extent our cells have evolved to recognize this structure and respond. Just not potently enough. The drug created by the MIT team can enter mammalian cells and is engineered to induce the cells to commit suicide if -- and only if -- they contain double-stranded RNA.