My understanding is also that your immune system is constantly fighting off cancer. Getting cancer is essentially a failure of the immune system. That doesn't necessarily contradict what they described in the paper. The immune system could fail because there were finally too many combined hits to fight it off. The immune system could fight off the first few "hits" but become overloaded or worn out or confused over time. There are many factors attacking the immune system: diet, exercise, environment and age also play a factor.