text: Viral genes and host genes that are required for viruses to replicate or enter the cell, or that play an important role in the life cycle of the virus are often targeted by antiviral therapies. RNAi has been used to target genes in several viral diseases, such as the human immunodeficiency virus (HIV) and hepatitis. In particular, siRNA was used to silence the primary HIV receptor chemokine receptor 5 (CCR5). This prevented the virus from entering the human peripheral blood lymphocytes and the primary hematopoietic stem cells. A similar technique was used to decrease the amount of the detectable virus in hepatitis B and C infected cells. In hepatitis B, siRNA silencing was used to target the surface antigen on the hepatitis B virus and led to a decrease in the number of viral components. In addition, siRNA techniques used in hepatitis C were able to lower the amount of the virus in the cell by 98%.
  RNA interference has been in commercial use to control virus diseases of plants for over 20 years (see Plant disease resistance). In 1986–1990, multiple examples of "coat protein-mediated resistance" against plant viruses were published, before RNAi had been discovered.  In 1993, work with tobacco etch virus first demonstrated that host organisms can target specific virus or mRNA sequences for degradation, and that this activity is the mechanism behind some examples of virus resistance in transgenic plants. The discovery of small interfering RNAs (the specificity determinant in RNA-mediated gene silencing) also utilized virus-induced post-transcriptional gene silencing in plants. By 1994, transgenic squash varieties had been generated expressing coat protein genes from three different viruses, providing squash hybrids with field-validated multiviral resistance that remain in commercial use at present. Potato lines expressing viral replicase sequences that confer resistance to potato leafroll virus were sold under the trade names NewLeaf Y and NewLeaf Plus, and were widely accepted in commercial production in 1999–2001, until McDonald's Corp. decided not to purchase GM potatoes and Monsanto decided to close their NatureMark potato business.  Another frequently cited example of virus resistance mediated by gene silencing involves papaya, where the Hawaiian papaya industry was rescued by virus-resistant GM papayas produced and licensed by university researchers rather than a large corporation.  These papayas also remain in use at present, although not without significant public protest, which is notably less evident in medical uses of gene silencing.
  Gene silencing techniques have also been used to target other viruses, such as the human papilloma virus, the West Nile virus, and the Tulane virus. The E6 gene in tumor samples retrieved from patients with the human papilloma virus was targeted and found to cause apoptosis in the infected cells. Plasmid siRNA expression vectors used to target the West Nile virus were also able to prevent the replication of viruses in cell lines. In addition, siRNA has been found to be successful in preventing the replication of the Tulane virus, part of the virus family Caliciviridae, by targeting both its structural and non-structural genes. By targeting the NTPase gene, one dose of siRNA 4 hours pre-infection was shown to control Tulane virus replication for 48 hours post-infection, reducing the viral titer by up to 2.6 logarithms.  Although the Tulane virus is species-specific and does not affect humans, it has been shown to be closely related to the human norovirus, which is the most common cause of acute gastroenteritis and food-borne disease outbreaks in the United States. Human noroviruses are notorious for being difficult to study in the laboratory, but the Tulane virus offers a model through which to study this family of viruses for the clinical goal of developing therapies that can be used to treat illnesses caused by human norovirus.