Not every bacteria is an agent of disease. Viruses Viruses are the smallest agents of disease. Their size vary from 0.1 to 0.004 microns. Except for the large pocketvirus, these micro-organisms can pass through the smallest filter, which can retain bacteria and protozoa. The most common virus are hepatitis, yellow fever, and poliomyelitis.
Virus tend to adhere to particles in the water which can be filtered out.
Protozoa Protozoa, larger than bacteria, are one-celled animals varying in size from 10 to 100 microns. They can survive in insects or in the form of cysts when outside of the human or animal organism. Some protozoa such as amoeba, giardia and cryptosporidium are present everywhere. Others, such as malaria or of the sleeping sickness disease can only be found in the tropics and subtropics. The remainder is held as ice, or salt water and therefore not drinkable.