All higher living organisms are composed of cells. Each cell runs biological DNA-based programs. These programs are very different compared to programs on a com

All higher living organisms are composed of cells. Each cell runs biological DNA-based programs. These programs are very different compared to programs on a computer (it's much more about biochemistry than electrical circuits). These programs are supposed to make cells perform their tasks correctly. But over time, the DNA can be affected by chemicals or faulty copy processes. The DNA sequence changes and the program "code" of the DNA is affected as well. The cells with altered DNA will behave slightly differently and in many cases nothing bad happens. But sometimes the changes affect functionality that is important to control the behaviour of the cell. The thing is, a single cancer cell is not a problem. The problems really start, once the cancer cells creates a great number of copies of itself and disrupts the operations of the other normal cells. What makes cancer tricky, is that cancer cells still are derived from normal cells and they might look too similar to normal cells so that it is difficult for the immune system to spot and kill them. What is now the root cause? The root cause is errors in the DNA sequence that accumulate over time. This is a natural process. The cells have some DNA repair mechanisms, but they are not perfect. To fully cure cancer, you would need to correct all DNA sequence errors which is not practical. However, in practise, this is not what is needed. Instead, it suffices if the cancer cells are being killed. And our immune system is pretty good at this. But at some point, some cancer will have a faulty DNA seuence, with new program code, that helps it hide from the immune system and then we need the help of doctors to get rid of the cancer.