What are viruses and what is their peculiarity?

Viruses are submicroscopic infections of the agent. We cannot see them with a conventional light microscope – we have to use high-resolution microscopy, which is electron microscopy. They are contagious and cause disease. Their entire life cycle is based on infecting the cell, so they infect all cell life. They cannot exist on their own, they cannot replicate if they are not within another cell, that is, they parasitize. They infect all cellular life, that is, animals, plants and bacteria.