Scientists cultivate viruses in order to isolate individual pathogens, identify the vast variety of viruses, and learn the details of their unique structures and processes. Isolation of a virus means obtaining a pure sample of it. Because viruses require living hosts in order to replicate, their cultivation requires the use of live tissues, bird eggs, or bacterial cultures. Identifying viruses enhances the ability of scientists and researchers to detect them and can reduce the time required for the diagnosis of a viral infection. Being able to place a newly investigated virus into a known group of viruses allows scientists to more quickly understand its capabilities.
Viruses can be cultivated for the production of vaccines. Vaccination is the technique of exposing an organism to a form of a pathogen to develop immunity against it. Vaccines can incorporate a variety of viruses or virus products. Some vaccines use attenuated, or weakened, forms of live viruses. Examples are the chicken pox (varicella) vaccine and the measles, mumps, and rubella (MMR) vaccine. Killed or inactivated vaccines, such as the polio or whooping cough (pertussis) vaccines, incorporate a protein or other component of a virus. Polio, cultured in human embryonic cells, was the first virus cultivated without bird eggs or solid animal tissue. Flu (influenza) vaccines, which are modified and developed annually, are primarily produced by incubating the virus in chicken eggs. Both inactivated and live forms of the flu vaccine are manufactured. Medical researchers have also developed biosynthetic vaccines, such as the hepatitis B vaccine, which include substances mimicking viruses that are biochemically synthesized without culture.
Cultivation of viruses permits detailed and important research into how viruses work. Research offers insight into what chemical structures bind to host receptors, the molecular mechanism of pathogenesis that underlies symptoms, and identify new targets on viruses that could be used as the basis for new vaccines. A famous experiment conducted by Alfred Day Hershey and Martha Chase in 1952 revealed that only nucleic acids were necessary for replication of a particular bacteriophage. In 1955 Rosalind Franklin determined the full structure of the tobacco mosaic virus, the first virus discovered and the first studied in detail. The discovery of the enzyme reverse transcriptase by independent researchers David Baltimore and Howard Temin in 1970 launched the important development of antiviral drugs, which are now used to treat infections such as HIV. New viral diseases continue to emerge, such as the Ebola and SARS (severe acute respiratory syndrome) viruses, and are the focus of critical research efforts.More recently, viruses have been utilized in cancer treatment in a process called oncolytic viral therapy. An oncolytic viral therapy is a medical treatment that uses a modified virus to induce cell death (lysis) in tumors. To date, two modified viruses have been approved by the U.S. Food and Drug Administration (FDA) for cancer treatment. The first is a modified herpes simplex virus 1 (HSV-1) with two genes removed and one human gene added to its genome. The two removed genes make the virus unable to shut down the host cell's defense and invade the immune system. The added human gene encodes for granulocyte-macrophage colony-stimulating factor, which stimulates the immune system. Modified HSV-1 is used to treat melanomas under the name T-VEC. The virus is injected directly into the cancer cells, simulating the patient's own immune system to destroy the cancer cells. The second virus that has been modified to treat cancer is human immunodeficiency virus (HIV). HIV is a retrovirus, meaning that it uses the enzyme reverse transcriptase to generate DNA from an RNA template. HIV naturally targets T cells, the body's cytotoxic side of the immune system, to insert the viral DNA into the T cell genome. To treat cancer cells, scientists removed the pathogenic genes and added receptor genes so that the infected T cell would produce specific receptors to target malignant B cells that cause leukemia. This allows the body's T cells to target and destroy the cancerous cells. This treatment is FDA approved under the name CAR-T.