The only way to be sure that water is safe is to treat it yourself. Remember to be careful of ice cubes and any food that may have been washed in untreated water.
Bacteria:These are tiny organisms. There are many times and there not all dangerous. They range in size from 0.5 to 1.5 microns. The main ones in water are E. Coli (which causes 40% of travellers diarrhoea), dysentery, cholera and typhoid. All bacteria are killed by boiling or iodine or chlorine treatments. The filter size for bacteria should be 0.2 microns.
Viruses:These are micro organisms. They live in side your cells multiplying within them making them difficult to treat. These are the most common problem when treating water. Common viruses include hepatitis A and E (0.027 microns),polio (0.020 microns) and meningitus (0.20 microns). The best defense is to get the correct vaccinations before you leave. These are killed by boiling or iodine or chlorine treatments.
Protozoans: These are hard shelled waterborne single celled parasitic cysts. Sizes vary from 2 to 15 microns. Amoebic dysentery and giardiasis are protozoans. To eliminate them you have to micro filter the water.
Water filters strain the water through fine filters. Micro filter pore sizes are quoted as average, nominal or absolute. For piece of mind you should opt for the absolute size. The average size may have holes considerably larger than the quoted size and the nominal size means that 70% to 80% of organisms of the quoted size will be removed.
The finest filters will not filter out all viruses so they should be used in conjunction with chemical treatment.
In the process of cleaning the water they can block up so it is important to clean the filter regularly. Check you know how to do this before you leave home.