A virus is an infectious agent made almost entirely of protein and nucleic acids. Viruses range in size from 0.02 to 0.25 micrometer (approximately 0.000000007 in) and can only be seen using an electron microscope. They are non-living assemblages of protein molecules and either ribonucleic acid (RNA) or deoxyribonucleic acid (DNA) that can replicate themselves only within the cells of their hosts. Viruses are known to cause many infectious diseases in humans, animals, and plants. Examples include diseases such as chicken pox, measles, encephalitis, hepatitis, herpes, influenza, viral meningitis, mumps, rabies, and even the common cold.

Viruses were discovered at the end of the nineteenth century when scientists began to understand the role that microorganisms played in the spread of many diseases. In 1892, the Russian botanist Dmitry Ivanovsky, discovered that the virus causing mosaic disease in tobacco plants could not be separated by filtration. This observation suggested that the organism was...