Since the 1940’s naturally occurring viruses have been identified that innately prefer targeting and replicating in cancer cells – oncolytic viruses. Like all viruses, oncolytic viruses penetrate a host cell and “trick” it into replicating more of the virus until he hosts cell’s membrane ruptures, and the cell is destroyed. Oncolytic viruses preferentially replicate in cancer cells. Once all the cancer cells are destroyed, the virus is generally no longer able to replicate and is cleared from the body. In 1956 first human clinical trial was conducted. In 2015 the first oncolytic virus was approved by the FDA for treatment of melanoma.