Read the following passage and answer the question.

Influenza is caused by a virus that attacks mainly the upper respiratory tract-the nose, throat and bronchi and rarely also the lungs. The genetic make up of Influenza virus allows for both major and minor genetic changes. making them immune to existing vaccines. Three times in the last century, the Influenza virus have undergone major genetic changes, resulting in global Pandemics and large tolls in terms of both disease and deaths. The most infamous pandemic was "Spanish Flu" which affected large parts of the world population and is thought to have killed at least 40 million people in 1918-19. More recently, two other Influenza pandemics occurred in 1957 (Asian Influenza) and 1968 (Hong Kong Influenza).

The most infamous pandemic was:

Spanish Flu