None of the above really, though estrogen plays a role

I think it is definnitely a chicken and egg kind of question.  What I believe, like the late great Antoine Bechamp,  is that what leads to cancer and most other illness is poor cellular terrain which is caused primarily by improper nourishment and cell maintenance in combination with a compromised immune system and exposure to toxins. Poor cellular terrain and a compromised immune system is an open invitation for microbes to do their dirty work and the end result is often disease.

Think about it - we have always had microbes but we have not always had the toxins, radiation and smoking of additive laced cigarettes.  The rate of cancer across all age groups and types of cancer has increased hand in hand with man made toxins - especially if you consider radiation and smoking.  Around the turn of last century only a few people out of each 100 could expect to get cancer in their lifetime.  Today one in every 2 or 3 people can expect it.  Have microbes increased by that much?  Hardly.  If it were merely viruses or bacteria, then there is no logical reason for the increase.  We have much better hygeine and supposedly much better protection against microbes (though when it comes to vaccines I would have to say that we have actually once again increased the toxic load) - and yet the incidence and death rate for cancer has risen steadily since man began introducing unnatural chemical compounds into the air we breathe, soil, food we eat, water we drink and medications we take.

In regards to breast cancer, I do think that estrogen plays a role but basically if a woman lives a healthy lifestyle, consumes a healthy diet, gets plenty of essential nutrients, and gets plenty of vitamin D3 (from the sun as much as possible) as well as ample iodine and its vital co-factor selenium, then her chances of getting breast cancer are drastically reduced.