We've had this discussion on this forum before. Viruses can contribute to cancer and some viruses can cause cancer, but far and away the number one cause of cancer is toxins.
The late, great French Scientist Antoine Bechamp told us a century and a half ago that cancer, and most other illness, was caused by a combination of toxins - particularly long term exposure to toxins - and unhealthy cellular terrain. Viruses are one of many items that have been put forward as cancer causes which actually are cancer contributors because they make cellular terrain even more unhealthy.
Mankind has had viruses, bacterial infections, stress, fungal infections and other proposed cancer causes of cancer forever. What we have not had until the last century and a half, is all the industrial toxins and the toxic ingredients in our prepared foods.
The proof is simple: when western diet and manufacturing are introduced to less developed countries, their cancer rates begin to climb to match those of western countries. Such undeveloped countries typically have no shortage of viruses, the same as the have no shortage of other proposed cancer causes such as bacterial infections and fungal infections.
Neither is cancer caused by low oxygen or acidic pH. Those are actually part of the cancer process itself. As the cancer grows and spreads, the body becomes increasingly acidic. Cancer cells have low oxygen levels because they stop taking in oxygen to use for energy production.
Insofar as what cancer is, Andreas Moritz explained to us that cancer is a natural body defense mechanism that kicks in when cells are under attack (under stress from toxins). The result is a change in mitochondrial RNA instructions that results in cells reverting to a primitive form of respiration whereby cells stop taking in and utilizing oxygen and begin begin using glucose for respiration (energy production to produce ATP).
All of the many proposed causes sell no end of products and protoc0ls, but they aren't the primary cause of cancer.