This document provides an introduction and overview of virology. It discusses that virology is the study of viruses and the diseases they cause. Viruses are small infectious agents that can only multiply inside host cells. The document outlines the history of virology, including early discoveries of tobacco mosaic virus and poliovirus. It also gives reasons for studying viruses, such as some viruses cause disease in plants and animals, others are useful research tools, and virus studies have contributed significantly to biological knowledge.