What keeps water coming into a house from becoming contaminated?

There are several reasons the water in the pipe stays relitvely uncontaminatexd
1) Given the water treatment plant filters, cleans, and treats the water to rigorous standards
2) The water is stored at high pressure. Once it leaves the facility it is pumped at high pressure 60 -120 psi to distribute through the water system. You have a regulator at your home which dials the pressure down to 60 psi or so. The high pressure reduces the chance of contaminants leeching in from the outside.
3) While in the pipes and under pressure the water is not exposed to air which contains bacteria and other bugs.

Before we got Town water on my road, my home used a well for our water. Once the Town water was hooked up I had to install check valves and separators to prevent any of my water from accidentally mixing with the Town water. The system is regularly inspected, at $90 per pop to make sure the system is working correctly.
I use well water to fill my fish pond and water the garden. I use Town water for cooking, drinking, bathing, etc.

Lack of air, which brings bacteria.

The water in the pipes is anaerobic.

That said, when I come back from a trip, I run the kitchen faucet to clear it out anyway.

The problem area is the aerator on the faucet, the air make contact with whatever water is there. I have a friend that went down to get his mother summer place at the beach ready, dusting and cleaning. He turned on the facet at the kitchen sink and it sputtered, the next week he was in the hospital with _Legionnaires’ disease _ which the doctors thought he got from the sputter of water.

Thanks for the answers. One thing that occurs to me is that the water that flowed in the pipes and aqueducts of the Roman Empire must have been full of bacteria, in addition to being contaminated by lead.

@LostInParadise It is possible the UV in sunlight and the constant aeration might have reduced the bacteria loading. However, it would only take one infection upstream to spread disease to overs downstream – think cholera.

The water coming from the city supply has been chlorinated to a level that it will still have residual chlorine far down the line. Over time that chlorine will be used up through biological demand. If you put it in a glass, the chlorine is still there, but since it has a lot of exposure to air, it will degrade the chlorine further, making it useless. Then any bacteria that are in the air or left in the water could start to grow.

I have a habit of running the water from the cold tap for a while after returning from holiday. The habit began in the bygone days of lead piping and probably isn’t worth doing nowadays but I still do it.

probably isn’t worth doing nowadays but I still do

I run the cold tap until the water IS cold before making my morning coffee. The habit comes from my grandmother (born 1899). I have no idea if there is an advantage.