On January 21, 1911, Francis Payton Laws of the United States published a medical report proposing that cancer tumors were caused by viruses. This was the first time in medical history.

The word "virus" is derived from Latin and originally referred to a toxin of animal origin. Viruses can proliferate, inherit and evolve, so they have the most basic characteristics of life, but there is no accepted definition of them. Originally used to identify viral traits, such as small size, generally invisible under light microscopy, can pass through filters that bacteria cannot pass through, cannot grow on artificial media, pathogenicity, etc., are still of practical significance. However, the characteristics that essentially distinguish viruses from other organisms are: ¢Ù genomes and protein shells containing a single nucleic acid (DNA or RNA), without cellular structure; ¢Ú nucleic acid is released at the same time as infecting cells or later, and then proliferates by nucleic acid replication, rather than by binary division; ¢Û strict intracellular parasitism. Viruses lack independent metabolic capacity and can only use the cell's biosynthetic machinery in a living host cell to replicate its nucleic acid and synthesize proteins encoded by its nucleic acid, and finally assemble into complete and infectious viral units, namely virions. Viral particles are the main form of virus transmission from cell to cell or from host to host.

Before viruses were discovered, people had already begun to unconsciously use viruses to serve humanity. In China, around the 16th century, healthy people were inoculated with the slurry from the abscesses of smallpox patients to inoculate them. Around the same time, Dutch growers infected tulips with viruses by grafting to produce beautiful broken-colored flowers; in 1796, E. Jenner invented the cowpox vaccine; in 1885, L. Louis Pasteur pioneered the rabies vaccine.

In 1892, Ivanovsky discovered that tobacco leaf juice with tobacco mosaic disease retained its infectivity after passing through a filter that blocked bacteria; in 1898, M. W. Beijerinck discovered this fact again and pointed out that the disease was caused by a class of pathogens different from bacteria. This is the beginning of understanding viruses. Later, it was discovered that many human, plant and animal diseases were caused by viruses. In 1898, F.A.J. Loeffler and P. Vorosch discovered the foot-and-mouth disease virus of cattle; in 1915, F.W. Tevot and in 1917, F. Errer discovered bacterial viruses, namely bacteriophages.

Since the 1930s, the physicochemical properties of viruses have been explored. M. Schlesinger purified bacteriophages and pointed out that they are composed of proteins and DNA. In 1935, W. M. Stanley obtained the crystallization of tobacco mosaic virus. In 1936, the virus was first seen under an electron microscope as a rod-shaped particle. Many viruses have been purified one after another, and their morphological structure and chemical composition have been studied, providing a basis for virus classification.

Due to the simple structure and components of viruses, and the fact that some viruses are easy to cultivate and quantify, viruses have always been an important material for molecular biology research since the 1940s. In the late 1930s, a group of scholars represented by M. Delbrück began to use the T-even phage of Escherichia coli to study its replication and genetic mechanisms, laying the foundation for molecular genetics. In the 1970s, the focus of research gradually shifted to animal viruses. Important advances in the development of molecular biology, such as the confirmation that DNA and RNA are genetic materials, the formation of the triplet code theory, the elucidation of nucleic acid replication mechanisms, the proposal of the central rule of genetic information flow, the discovery of reverse transcriptase and gene overlap and discontinuity, etc., as well as the rise of genetic engineering and the development of carcinogenesis theory, are almost all related to viruses. The primary structure analysis of some proteins and nucleic acids is often first completed using viruses as materials. In turn, molecular biology research has promoted the understanding of virus structure, replication and inheritance, allowing virology to develop into an independent branch of discipline.

In terms of practice, the study of viruses has made important contributions to the prevention and control of viral diseases in humans, plants and animals. The development of virus vaccines has provided effective measures for the control of human diseases (such as smallpox, yellow fever, polio, measles, etc.) and livestock and poultry diseases (such as rinderpest, swine fever, chicken Newcastle disease, etc.); due to the use of comprehensive control and disease-resistant breeding measures, potato degenerative diseases, wheat soil-borne mosaic disease, cabbage turnip mosaic disease and other agricultural diseases have been effectively controlled; the use of insect viruses as insecticides is also being vigorously carried out and has entered the practical stage.