The micropores that constitute the Black Berkey filters are so fine that they stop microparticles such as viruses and bacteria from passing through. Even food colouring cannot pass through, and the filtered water becomes crystal clear! Viruses and bacteria are eventually trapped in these extremely fine 'tortuous paths' (in the range of a few microns), where as the smaller water molecules (H2O) are not. This method efficiently distinguishes contaminants from purified water. For more details on filtration capacity, you can consult the complete test results.