Viruses are not considered to be living organisms because they cannot grow or replicate on their own. The most general structure of a virus is a nucleic acid encased in a protein coat. They act like living organisms when they infect a living host cell and use the cell's machinery to reproduce themselves. Scientists in these fields discovered simultaneously that the agents of these diseases were not filterable out of solutions by the filters that typically caught bacteria. A virulent virus is one that takes over the cell's machinery, makes more viruses, and ruptures or lyses the cell, releasing the new viral particles.
Temperate viruses infect a host cell and become established in the cell's own genome.
It is difficult to treat a viral infection of either type because the virus becomes so well integrated with the host genome that inhibiting the virus also inhibits the host cells. In bacterial infections, on the other hand, one can generally kill prokaryotic bacteria with little effect on the eukaryotic host cells. Generally an electron microscope is required to see an actual viral particle, so the approximate size range is 17 to 1000 nanometers. A bacteriophage is a virus that infects a bacterium. The tail retracts, and the tube within it penetrates the bacterial cell wall, permitting the T4 nucleic acid to enter the bacterium.