A virus is an infectious particle that is not living and can only function inside a living cell. Most viruses are too small to be viewed with a compound light microscope and must be viewed with an electron microscope.
Similarities to Prokaryotic and Eukaryotic Cells
Viruses contain nucleic acid (either DNA or RNA) much like prokaryotes and eukaryotes, however, viruses need a host cell in order to proliferate.
Differences from Prokaryotic and Eukaryotic Cells
Viruses are unable to reproduce independently.
Examples of diseases caused by viruses:
Source: Concepts in Biology Twelfth Edition Eldon Enger, et al.
This audio will introduce viruses.