The proper definition of a virus is an organism that is found in all life forms, including humans, animals, plants, fungi, and bacteria. Viruses can cause the flu, the cold, cholera, AIDS, HIV, etc. For a virus to live, all it really needs another living organism.
How a virus reproduces is similar to yeast molds. A single virus multiplies, and spreads into more and more viruses.
The first virus that was ever discovered was the tobacco mosaic virus. The first successful transmission of a virus was conducted by Adolf Mayer in 1880. Mayer was unable to isolate the virus so, Dimitri Ivanofsky extended Mayer's observation and reported that the tobacco virus was able to pass through a porcelain filter which was known to block the passage of bacteria. Ivanofsky also failed to isolate the virus. Martinus Beijerinck repeated the experiment and found out that the virus could reproduce. Beijerinck referred this new microorganism as a contagious living liquid.
The bacteriophage can make holes in a culture of bacteria. It was discovered by Fèlix H. d'Hèrelle. This was the solution for killing virus cells. It was a great discovery.
So, as you might have learned from this, viruses are very deadly to every living thing that lives on the face of the Earth.