The document outlines the development of virology from the late 19th century to present day. Some of the key events and discoveries discussed include: - In 1892, it was discovered that tobacco mosaic disease and foot and mouth disease could be transmitted via bacterial-filtered fluids, suggesting the existence of a "new" infectious agent smaller than bacteria called a virus. - In the early 1900s, it was established that viruses can only propagate within living cells and are obligate intracellular parasites. - In the 1930s, it was shown that viruses are particulate and the virus particles were termed "virions". Tobacco mosaic virus was also purified. - Three basic groups of viruses were identified: RNA viruses, DNA viruses