Anti-virals are a class of pharmaceuticals used in the treatment of viruses. Unlike antibiotics, most anti-virals are only effective against a very narrow class of viruses. In addition, their side effects are usually more severe. Some drugs in this class that are frequently mentioned on the show are amantadine, acyclovir and AZT. More broadly, they also include such treatments that are effective against viral infections, but enhance the body's natural defences to viruses, such as interferon and vaccines.
The chief problem in developing an anti-viral drug is, again unlike bacteria, they only replicate within the cells of the host (unlike bacteria, which are cells of their own). As such, most substances that destroy viruses will also kill normal cells. Anti-virals take advantage of certain biological differences between viruses and normal cells.
In addiiton, viruses replicate so quickly that they can develop resistance to drugs much faster than bacteria can. Long term use of anti-virals for chronic infections such as HIV usually result in the drug becoming ineffective.
Anti-virals are very new, first having been developed in the 1960s to deal with herpes.