Microorganisms are any microscopic form of life; a form of life too small to be seen by the naked eye such as viruses, bacteria, fungus, mold, parasites, protozoa, yeasts, and similar organisms, esp. such as those that are supposed to cause infectious diseases.
The Epstein–Barr virus, formally called Human gammaherpesvirus 4, is one of the nine known human herpesvirus types in the herpes family, and is one of the most common viruses in humans. EBV is a double-stranded DNA virus. It is best known as the cause of infectious mononucleosis.