It has been a very long time since people have started boiling their water in order to cook food, but it is only relatively recently that people have boiled water to make is safe for human consumption.
Before the practice of sterilizing water through boiling was common, many people were exposed to all sorts of different and terrible bacteria and parasites. Some of these parasites include, guinea worm, cryptosporidiosis, schistosomiasis, amebiasis, and giardiasis. These are some nasty little creatures that can cause diarrhea, pain and even death if they get the opportunity to make you their host.
Properly boiled water will easily kill off these parasites and make your water a lot safer for you to drink. There are also bacteria in water, boiling your drinking water will easily remove these as well. You can also bleach your water clean, but I do not suggest that you do this unless you know exactly how much bleach to use, or you can chlorinate your water to remove bacteria.
To properly ensure that your drinking water is properly boiled to ensure that all parasites and bacteria have been completely killed off, bring your water to a roiling boil for at least 5 minutes. Any less than this temperature and time means that you still might have a contaminated source of drinking water.
Although boiling your water to make it safe to drink is an excellent way to prevent a horrible death, you are also going to want to filter your drinking water before you consume it. You would be surprised how many things can get past the boiling process and into your system. There are several different and harmful chemicals that will not be removed by boiling, these have to be dealt with by filtering your drinking water or distilling it.
If you are extremely nervous about the condition of the water that you are about to consume, distillation is by far the most effective of getting clean water. This process does not remove the harmful elements from your drinking water. What it actually does is remove the clean drinking water from the harmful elements. It is a complicated and relatively expensive process, so make sure that it is absolutely necessary before doing it.