HPV is found in the cancer cells of more than half the patients with certain types of cancer, suggesting that it may play a role in causing the cancer. The virus makes proteins in the tumor cells that should allow the diseased cells to be recognized and killed by a part of the body's own immune system called T cells. Unfortunately, tumors are able to avoid being killed by making molecules that turn off these T cells. So, the cancer cells infected by HPV are able to hide from the body's immune system and escape destruction.