Total Coliform is a family of bacteria and E. coli is a specific type of Coliform.
They are considered “indicator bacteria”. The EPA says if you have any Total Coliform and/or E. coli (“one or more colony forming units) that the water should be treated. Therefore, the test we use (which is also used by the NC State Lab of Public Health) is an absent/present test. If it is present, the water should be treated.
Total Coliform and E. coli can get into wells/springs and build up slowly over time. For people whose immune systems work properly, their body will build up a resistance to the bacteria.
But if someone comes in from city water, or if someone has immune system issues, it can make them sick as their immune system does not know how to fight it.
There are many items that can cause a test to be positive: wells that have been sitting unused tend to be positive; weather can affect it (especially lots of rain); issues with the well like a crack in the casing or an open vent hole); and sometimes it just happens.