Abstract

A virus is a small infectious agent that replicates only inside the living cells of an organism. Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea. In evolution, viruses are an important means of horizontal gene transfer, which increases genetic diversity in a way analogous to sexual reproduction. Influenza (Including (COVID-19), is an infectious disease caused by an influenza virus. Some viruses especially smallpox, throughout history, has killed between 300-500 million people in its 12,000- year existence. As modern humans increased in numbers, new infectious diseases emerged, including SARS-CoV-2. We have two groups of virus, RNA and DNA viruses. The most brutal viruses are RNA ones like COVID-19 (Sars-CoV-2 [1] Introduction: Coronaviruses are a group of viruses that cause diseases in mammals and birds. In humans, coronaviruses cause respiratory tract infections that are typically mild, such as some cases of the common cold (among other possible causes, predominantly rhinoviruses), though rarer forms can be lethal, such as SARS, MERS, and COVID-19. Symptoms vary in other species: in chickens, they cause an upper respiratory tract disease, while in cows and pigs they cause diarrhea. Coronaviruses constitute the subfamily Orthocoronavirinae, The genome size, coronaviruses ranges from approximately 27 to 34 kilobases, the largest among known RNA viruses..