My experience suggests no one knows what caused any particular person's BE, and, as someone else said, there's likely not just one causal factor. When I was first diagnosed, the pulmonologist practically insisted that I must have be born prematurely. I was not, and he came close to calling me a liar. Others said my history of smoking was probably not a cause. One doc said perhaps the MAC caused the BE; seems to me the BE was a set-up for getting MAC (an opinion with which my current doc agrees); another said measles (which I had at age 4) could have caused it. My brothers smoked more than I, and one had a different lung disease, but neither had or has BE. So, who knows? I'm not sure it matters how any of us got the condition.