Viral diseases are much harder to treat than bacterial ones. Antibiotics do not work against viruses, because viruses have no peptidoglycan cell wall, no 70S ribosomes and no metabolism of their own for antibiotics to target. Giving antibiotics for a viral infection only increases the risk of antibiotic resistance in bacteria.The main difficulty is that viruses replicate inside host cells using the host's own machinery. A drug that stops viral replication may also damage the host cell. Useful antiviral drugs therefore target molecules or stages that are unique to the virus:• Blocking entry: entry inhibitors stop the virus binding to or fusing with the host cell.
• Blocking genome copying: reverse transcriptase inhibitors such as zidovudine (AZT) stop HIV making DNA. Aciclovir is a nucleoside analogue that is activated by a herpes virus enzyme and then stops viral DNA synthesis, so it acts mainly in infected cells.
• Blocking protein processing: protease inhibitors stop HIV cutting its proteins into working forms, so new virions are not infectious.
• Blocking release: neuraminidase inhibitors such as oseltamivir (Tamiflu) stop new influenza particles being released from the cell.Antivirals slow viral replication rather than kill the virus; the immune system still has to clear the infection. They also cannot reach latent viruses, such as the HIV provirus.Because RNA viruses mutate quickly, resistance to a single drug develops rapidly. HIV is treated with combination therapy (antiretroviral therapy, ART) of several drugs acting on different targets at once. A virus would need several mutations at the same time to resist them all.For many viral infections, such as colds, treatment is simply relief of symptoms, rest and fluids.
• Blocking genome copying: reverse transcriptase inhibitors such as zidovudine (AZT) stop HIV making DNA. Aciclovir is a nucleoside analogue that is activated by a herpes virus enzyme and then stops viral DNA synthesis, so it acts mainly in infected cells.
• Blocking protein processing: protease inhibitors stop HIV cutting its proteins into working forms, so new virions are not infectious.
• Blocking release: neuraminidase inhibitors such as oseltamivir (Tamiflu) stop new influenza particles being released from the cell.Antivirals slow viral replication rather than kill the virus; the immune system still has to clear the infection. They also cannot reach latent viruses, such as the HIV provirus.Because RNA viruses mutate quickly, resistance to a single drug develops rapidly. HIV is treated with combination therapy (antiretroviral therapy, ART) of several drugs acting on different targets at once. A virus would need several mutations at the same time to resist them all.For many viral infections, such as colds, treatment is simply relief of symptoms, rest and fluids.