The most common killers in the early 1900s, were pneumonia, tuberculosis, and diarrhea (< this one really surprised me!) About a third of the people died from these diseases. This is probably because of the lack of hygiene and medicine. It doesn’t say much about Mattie’s mother’s death, but I think that she died from one of these diseases. It seems pretty plausible. In the 1900s, there was also a huge problem contaminated food and water, so chances are she could have eaten bad food/water and gotten a disease that was incurable at the time. There was also a sewage disposal issue, which harbored bacteria of many kinds. Since she was at home all the time, there was probably a higher chance of her catching something from the air.
The sites where I got this amazing info... Check it out!! especially the second one.
http://stay-healthy-enjoy-life.blogspot.com/2008/04/causes-of-death-2005-versus-1900.html
http://www.uky.edu/Classes/PHI/350/cod.htm