For various reasons, the genetic material of the virus has changed. Many chemical and physical factors can be used to induce mutation, such as nitrite, hydroxylamine and high temperature. In addition, viral mutations sometimes lead to drug resistance. Viruses mutate easily. Besides virus-like, virus can be said to be the simplest member in life. Its genetic code or genome is mainly concentrated on the nucleic acid chain, and any change in this nucleic acid chain will affect the characteristics of its offspring. In fact, the virus genome is not static during its proliferation, but has been mutating automatically. Most of these mutations are fatal, and only a few can survive. Because the virus must reproduce millions of times in an infection, it has the opportunity to mutate.

So, a virus is not genetically homologous at the population level, so what about a virus? Species? It is not a taxonomic species in the strict sense, but should be called a quasi-species. The natural mutation of virus is very slow, but this mutation process can be accelerated under the stimulation of powerful external factors. Virus mutation refers to the chemical change of nucleic acid base sequence in genome, which can be a change of one nucleotide, or a deletion or translocation of hundreds or thousands of nucleotides. The natural mutation rate in virus replication is 10-5 ~ 10-8. Temperature, radiation, 5- bromouracil, nitrite and other physical and chemical mutagens can improve the mutation rate. Different from the original wild-type virus, the mutant has changed in virus virulence, antigen composition, temperature and host range.

Conditionally lethal mutant refers to a virus that can grow under specific conditions after mutation, but cannot reproduce under the original conditions and is killed. The most important one is the temperature-sensitive conditionally lethal mutant (ts strain), which can proliferate at a specific temperature (28 ~ 35℃), but not at an unspecified temperature (37 ~ 40℃), while the wild type can proliferate at both temperatures. Obviously, at an unspecified temperature, the protein encoded by the mutant gene lacks its proper function. Therefore, most ts strains are attenuated strains. Ts strains have been isolated from a variety of animal viruses, and strains with good genetic stability have been screened out for preparing vaccines against hepatic alkalosis, such as influenza virus and spinal gray anti-inflammatory virus ts strains.

Used in genetic engineering, genetic engineering is to transfer the gene of one organism, that is, the DNA fragment carrying genetic information, to another organism, and combine it with the DNA of the original organism to realize the transfer and recombination of genetic traits, so that people can directionally control, control and change the variation and inheritance of organisms. Virus genetic engineering is developing in two directions: first, the gene encoding virus surface antigen is transplanted into plasmid, and a large number of surface antigen substances are produced in Escherichia coli to prepare vaccines or diagnostic antigens. For example, the DNA fragment encoding hepatitis B virus surface antigen has been expressed in yeast, and the vaccine is being observed in human body; The second is to explore the possibility of virus as a carrier of genetic engineering, and bring the required foreign genes into human body or branches, so as to treat human genetic diseases or create new animal varieties.