No matter how much we try to purify or filter water, it's virtually a guarantee that we'll never get every bit of bacteria out of a glass that we intend to drink.
An average glass of water taken from the sink is estimated to have almost 80,000 bacteria per milliliter, and there are still another 2,000 bacteria that the water passes through when it moves through the plumbing pipes of a water system in a home.
Most of this bacteria aren't harmful to humans, and it passes through the body safely, but sometimes, weaker individuals, such as the elderly or young children can be affected by bad bacteria that can cause problems down the line. This is why it's recommended that you use some kind of filtration device like one that you can affix to your faucet.
The next time you pick up a glass of water, you'll probably be reminded that it's full of bacteria that you can't see, but just remember that not all bacteria is bad, and most of it is harmless.