In theory, any cell in the body can undergo a malignant transformation and become cancerous. There are many different reasons why this might happen. The most common one is that defects and mutations in the genes interfere with the regulation of the cell cycle. This then causes the cell to begin multiplying unchecked and very quickly. Cancerous cells can also move into other parts of the body and make new tumours grow there.
Some risk factors are known to increase the likelihood of cells becoming cancerous, and we can protect ourselves from these. However, they don’t play a role in every form of cancer. Many tumours are caused by random errors in cell division that we can’t protect ourselves from. As a general rule, we can reduce our chances of developing cancer by leading a healthy life. The German-language website of the German Cancer Research Center offers a good overview of cancer risk factors on the page entitled Risiken vermeiden – Krebs vorbeugen (Avoiding risks – Preventing cancer).
Saskia Blank of the Ideas 2020 team answered this question.