Every time you drink from your bottle, it comes into contact with your hands and saliva, which carry different types of bacteria.

Even if you only drink water, bacteria still spread inside the bottle.
If you don’t clean your bottle properly, an unpleasant smell quickly develops due to bacterial growth.

The bacteria start producing a sticky substance that helps them attach to surfaces, forming an invisible, slippery layer known as biofilm.

From there, mold can develop quite quickly, making the bottle unsuitable for use.

An unwashed bottle can also contaminate your drink, which may in turn lead to various health issues (e.g. stomach issues).