Question

Influenza was known in humans well before the 1918-1919 pandemic. why do you think that no human influenza viruses are indicated on the tree before about 1918?

Answer 1

The 1918-1919 influenza pandemic was caused by a new strain of the H1N1 virus resulting from an antigenic shift. Prior to this event, influenza viruses in humans existed but were not as severe or deadly.

The conditions of World War I contributed to the rapid spread of the virus.

Step-by-step explanation:

The reason why no human influenza viruses are indicated before about 1918 is that the 1918-1919 influenza pandemic, also known as the Spanish flu, was caused by a new strain of the H1N1 virus.

This strain was the result of an antigenic shift, where avian and human viruses recombined to create a new virus.

Prior to this event, influenza viruses in humans did exist, but they were not as severe or deadly as the 1918 strain. The conditions of World War I, such as crowding and poor sanitation, greatly contributed to the rapid spread of the virus once it appeared, leading to the high mortality rate.