When a virus transforms a cell it often causes cancer by either altering the cells' existing genome or introducing additional genetic material which causes cells to uncontrollably replicate. It is rarely considered that what causes so much harm also has the capability of reversing the process and slowing the cancer growth or even leading to remission. Viruses transform host cells in order to survive and replicate; however, the immune responses of the host cell are typically compromised during transformation making transformed cells more susceptible to other viruses.
The idea of using viruses to treat cancers was first introduced in 1951 when a 4-year-old boy suddenly went into a temporary remission from leukemia while he had chickenpox.  This led to research in the 1990s where scientists worked to create a strain of the herpes simplex virus strong enough to infect and transform tumor cells but weak enough to leave healthy cells unharmed.  Treating patients with viral transformation has the possibility of treating patients more safely and more effectively than using traditional methods, such as chemotherapy.  Viruses used in the treatment of cancer gain strength and increase their effectiveness as the multiply in the body while causing only minor side effects, such as nausea, fatigue, and aches.