Our knowledge of asymptomatic infection began with the famous typhoid Mary. Mary mallon is an American chef. She has typhus in her body, but she is not ill. She has always been healthy, but the bacteria she excretes may be transmitted to others and cause diseases. As a cook, she is thought to have infected at least 49 people, three of whom died. She finally died in 1938, at the age of 69, of pneumonia, not typhoid fever. Asymptomatic infection widely exists in nature, including human beings.
Asymptomatic infection is widespread in the biological world, not just in humans. Coronavirus pneumonia-19 and its cousins, SARS and Middle East respiratory syndrome, as well as influenza, AIDS and Ebola, are diseases of human beings and animals, and pathogens are transmitted from animals to humans. These pathogens or their precursors live in some animals and can be transmitted to humans directly or through "bridges" between animals. These animals are called hosts of corresponding pathogens, and they are called main hosts (lower circle animals) and intermediate hosts (lower circle animals) respectively.
What about asymptomatic infected people? Why can't people get sick with viruses? At present, it is considered that the original hosts all carry microorganisms, but they cannot cause diseases, and they are asymptomatic infections, which can coexist peacefully and evolve together for a long time. For example, the original hosts of coronavirus pneumonia-19, SARS and MERS in novel coronavirus are bats, and they can all live in bats without causing diseases. Hantavirus, which attracted people's attention two days ago, is also a * * * virus to its natural host. It will infect some mice, but it will not cause diseases. These two viruses have evolved together for millions of years.