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 sickness 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.
For further information please go to The Center for Disease Control: