Coliforms are not one kind of bacteria, but many, and they can make you sick if ingested from drinking water. But most coliforms are harmless residents of soil and will not make people sick. Some strains of E. coli, the most common fecal coliform bacterium (usually living in animal fecal material) may cause disease. Some E. coli strains found in food have been lethal. Their presence should be taken very seriously.
Your immune system also determines whether on not you will become ill when exposed to bacteria. People can become accustomed to the natural bacteria in their water while guests used to other water conditions may have some discomfort or diarrhea when they drink the same water. Immune-compromised individuals may become ill under circumstances where people with normal immune systems would not, and should be very careful of the water they drink.