A virus is a tiny package of genetic information (DNA or RNA) inside a protein shell. The package is inserted into the cell of another living organism so that it can reproduce and use the host’s cell machinery to do so. Some viruses make people sick, but most do not. There may be trillions of harmless viruses living in your body.
Unlike bacteria, viruses lack the necessary machinery to survive independently and therefore must seek out plant, animal, or bacterial cells to infect in order to reproduce. The virus attaches itself to the cell by recognizing specific receptor molecules on the host cell’s surface. The proteins on the outside of the virus, called capsids, also bind to receptors and help in the attachment process.
Once inside a host cell, the viral nucleic acid is copied (or reproduced) by using the host’s enzymes and the host cell’s cellular machinery. The viruses are inert outside the host cell, but once they are inside they start producing thousands of copies of themselves very quickly. This burst of reproduction is referred to as the lytic cycle.
While a large number of viruses cause diseases in humans, the growing knowledge about how they replicate and infect has opened up new possibilities for gene therapy. Viruses that do not make people sick can be used to deliver healthy genes into cells to replace defective ones. The gene-delivering virus AAV is an example of this. This technique, which relies on a “helper” virus to enter the cell and introduce the new DNA or RNA, shows great promise in treating diseases such as muscular dystrophy, spinal cord injuries, and blindness.