Re:

 So in your opinion cancer viruses are a common cause of cancer. Well not exactly according to "Cancer Research UK" for they say what happens is that the virus can cause genetic changes in cells that make them more likely to become cancerous.

These cancers and viruses are "linked" but not the "cause" of cancer. What happens is that the virus can cause genetic changesin cells that make them more likely to become cancerous, there is a difference. 

It is estimated that 80% of the population is infected with the cytomegalovirus (CMV).  But not everyone with CMV gets CMV retinitis.  

Point is that viruses do not cause disease in everyone.  A large part of the reason is that viruses have a hard time infecting and/or being pathogenic in people with healthy immune systems.  Take those same viruses in people with severely compromised immune systems and they can and will cause diseases, including cancers. Look in to the history of David "the bubble boy" as an example since he died from a viral induced cancer.  And if you believe that AIDS is caused by HIV then look up the fact that HIV is also a cancer virus (human T-cell leukemia/lymphoma virus type 3), which is why leukemias and lymphomas are so common in AIDS patients.

Unfortunately too many people here on CZ want to start arguing without bothering to research the real facts before responding.