Adenoviruses are known as the "common cold" virus and most individuals have had multiple infections during their lifetime. Oncolytic adenoviruses are adenoviruses that are modified so they cannot multiply and spread (known as replicating) properly in normal (e.g. healthy) cells, but instead, they infect and replicate very well in cancer cells. This strong replication leads to the death of the cancer cell. Oncolytic viruses have been evaluated in multiple clinical trials for cancer treatment during the past decade and been proven safe. It is common to have a fever the first day or two after virus injection since the immune system will react to the virus infection. The immune system can also kill cancer cells but to do so it needs to be properly stimulated. Oncolytic viruses alone do not seem to be strong enough to activate clinically relevant anti-cancer responses. However, it is thought that if additional immune system stimulators are added to the oncolytic viruses they may be able to result in clinical relevant antic-cancer responses.