When and by whom were the virus discovered?

In 1898, the Dutch microbiologist Martinus Beijerinck repeated experiments and became convinced that a filtered solution contained a new form of infectious agent (other than bacteria). He observed that the agent multiplied only in cells that were dividing, but as his experiments did not show that it was made of particles, he called it a contagium vivum fluidum (soluble living germ) and re-introduced the word virus.
I hope that helps. :-)