ACC only covers you if you are injured and doesn’t cover an injury which can be linked to an illness or a degenerative condition. For example, if you fall and smash your knee, and you need knee surgery, doctors could discover that you have some arthritis in your knee. This can complicate both the surgery and the income you were expecting to get from ACC. However, if you have private medical insurance, you know exactly where you stand and what is covered. After the knee injury, if a condition is discovered which you didn’t know about, it wouldn’t affect your claim in any way.