What's the difference between "normal flora" e. coli and the dangerous kind? or do we simply get sick from flora that isn't our own?

Both can happen: The E. coli strains we all carry in our intestines usually are harmless and even benficial, but even these sometimes cause urinary tract and other infections, sometimes dangerous ones. However, the food borne types that orginate in other animals sometimes produce unique toxins not present in our own natural strains. These are the more dangerous ones that cause most serious food borne outbreaks.