Once we put the bottle in our mouth, all of this can 'backwash' into the remaining water, contaminating it. In addition, our saliva, which also carries bacteria, can do the same. So, if it's allowed to incubate for hours, it could potentially contaminate the water, and make you ill by reintroducing that bacteria. So, once you put your lips to the bottle, you should consume the bottle in one sitting, then discard it.
Of course, since it's your own bacteria, it is unlikely that you will get sick. After all, many people sip from used drinking glasses, mugs and bottles without any ill effects. But it's certainly not advisable that you share your bottle with someone else. Furthermore, people with a reduced immune system, such as transplant patients, those undergoing chemotherapy, or people living with HIV/AIDS, should never be exposed to contaminated water.
In case you're wondering, it makes no difference whether it's bottled or tap water. It's a common myth that bottled water is cleaner than tap as both have to meet the exact same hygiene standards. Also, bottled water is drawn from the main water supply.
What about water that is left in places such as your car? Water that is left in the sun will heat up, making it the perfect breeding ground for bacteria, particularly when you have already drunk from it. So, placing the bottle under your seat may reduce the heat a little, causing the bacteria to grow.
There are also some kinds of plastic bottles that contain BPA or similar chemicals which can find their way into the water, particularly when exposed to sunlight. Research indicates that BPA can be linked to a number of health problems that affect the brain and behavior.
However, according to the FDA, the level of BPA transference is within safe limits. Using a BPA-free bottle, however, would completely eliminate these chances. However, it would not eliminate the growth of bacteria, particularly if you're using a metal bottle which heats up quickly, thus encouraging germs to multiply.
Nevertheless, staying hydrated is good for our health, so always be sure to recognize the signs of dehydration. Just avoid putting your mouth to the bottle, pour it into a cup and directly to your mouth instead.