The novel talks of e-coli and typhoid spreading, as well as concerted terrorist attacks.
So very basically, how would these diseases be spread?
Lack of sanitation from human waste getting into fresh water supplies and a lack of hand washing or washing hands in contaminated water where human waste was deposited.
When someone urinated or defecate into fresh water it is contaminated until sanitized by any number of means like boiling or chemical sanitation or filtered.
Latrines (hole in the ground) need to be a minimum of 90 meters away from any fresh water supply.
When the first settlers came to America to establish a colony one of their priorities was setting up a brewery because drinking fresh water was not ever done. (They certainly didn't)
All of the wells, streams, rivers and etc in Europe in all the villages contained fecal contamination to some degree. Mostly because of livestock and human waste. Street Gutters would literally flow into the town's water supply. (Which is where chamber pots and etc were emptied....not to mention horses were regularly used)
And if city sewer services are defunct, not working etc....they tend to overflow back into the river or wells from which your water supplies come from.