How many years ago did swine flu break out?

19 18 ~ 1920 first appeared in the eastern United States in 19 18, and it was popular in the French army in April of 19 18, and then spread rapidly. This epidemic is called the biggest plague in human history, and the total death toll is estimated to be about 20 million.

The variation of influenza virus is the most important, which is often closely related to the worldwide epidemic. Generally speaking, the antigenic variation of influenza virus refers to the change of H and N antigenic structures, and small variation (quantitative change) often occurs within subtypes, which is called antigen drift.

Although it is only a slight mutation, it can make the virus easily escape from the host's immune system. Compared with the previously discovered strains, if the antigenic determinant (epitope) of hemagglutinin molecule mutates, it is considered that the new strain is a heterologous variant of the previous strain, which has epidemiological significance and can cause the epidemic of influenza.