A filter if used properly may remove 99.9% of Protozoa (Crypto & Giardia) in the water, yet smaller contaminants such as bacteria and viruses can pass through allowing contamination and possible illness as a result.
A purifier must remove 99.9% of Protozoa, 99.99% of Viruses and 99.9999% of Bacteria.
These numbers are important due to the size of various contaminants and how many of each can make you ill. Protozoa are anywhere from 1 to 100 microns in size. Bacteria are between 0.1 and 1 micron. Viruses range from 0.005 to 0.3 micron. There is not a filter small enough to filter out the viruses.
For this reason a Bio-Sand filter although a vast improvement over many other available methods cannot protect you from all viruses and bacteria.
Populated areas of developing countries have a high risk of protozoa contamination, a moderate to high risk of bacteria contamination from both human and animal carriers and a moderate to high risk of virus contamination.
So for the safest drinking water in these countries, you should focus your search on water purification systems.
For further information please go to The Center for Disease Control: