Question: How does breast cancer usually occur

James Brown answered on 10 Nov 2022:

Cancer cells happen when normal cells gain errors or changes that alter how the work and grow. If you get changes in specific genes (or have enough small changes in multiple genes over time), this can allow a cell to start growing without control (which is cancer).

Its for this reason that cancer is also a disease of old age, as you collect more changes as you grow older (and as a society, we are living longer).