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Unlike bacteria, which can be killed relatively simply with antibiotics such as penicillin, viruses are not so easily terminated. In virtually all cases of serious viral infection, antiviral therapy is administered less with the intention of destroying the virus than simply retarding its growth and forcing it into dormancy. Antiviral therapy is considered a more complex science than antibiotics, because the nature of viruses and the way they replicate make it comparatively difficult to develop drugs that target the pathogen, but do not do equal damage to healthy cells.
Vaccines have been effective at preventing viral infection since the 18th century. Until the 1960s, however, no way existed to combat a virus once infection had taken place. The first successful efforts were made against the herpes virus — strains of which, in addition to cold sores and genital herpes, are also responsible for illnesses like chicken pox and shingles. Progress fighting viruses continued in a hit-or-miss fashion until the 1980s, when breakthroughs unraveling the genetic codes of viruses gave researchers a much clearer road map to developing new, more refined antiviral therapy treatments.
Some of the other viruses generally treated with antiviral therapy include human immunodeficiency virus (HIV), hepatitis, and influenza. With HIV treatment in particular, billions have been invested to develop effective antiviral therapy to subdue the virus, which eventually leads to the incurable condition known as Acquired Immune Deficiency Syndrome (AIDS).
The two most common kinds of antiviral therapy involve substances known as nucleotide analogues and interferons. Nucleotide analogues attempt to suppress a virus's ability to enter a healthy cell and take it over for the purpose of replication. They appear to viruses to be a normal nucleotide, which is a substance they require for replication, but effectively sabotage the replication process and cause the virus to stop duplicating itself. The first drugs developed to combat herpes and HIV were this type of antiviral.
Interferons, by comparison, are proteins that work to prevent spread to other cells and enhance the body's own immune system in fighting a virus. They too inhibit replication of a virus, but also alert naturally-occurring immune cells, such as macrophages, to the presence of the target virus. Common side effects from interferon use are weight loss and a general feeling of malaise, which tends to limit the degree to which it can be used.
Though viruses are fundamentally different from bacteria, anti-viral drugs and antibiotics both suffer increased resistance over time, as both bacteria and viruses mutate over the course of generations to survive a given type of treatment. This is due in large part to the fact that most antivirals must be targeted against a specific virus. Only a few types of interferons may be used against a wide range of viruses. A growing strategy in the field of antiviral therapy is to couple the use of nucleotide and interferon treatments together, in an attempt to nullify the advantage of a virus to develop resistance to a particular type of treatment.