Present year-round, emerges during different seasons in different parts of the world.

It causes illnesses that range in severity. Often people recover without treatment, but it means they mostly will not be able to work during illness. Sometimes the illnesses lead to hospitalization. However, since so many people are infected every year, this is still a large number of people. The estimated number of deaths is ~400.000 per year worldwide.

Source: WHO

Relatively rare, there have been 3 notable pandemics in the 20th century and one (the ‘Swine flu’) in the current century (2009).

The impact of a pandemic is highly dependent on the level of transmission and disease. The most devastating pandemic was the Spanish flu in 1918/1919, which is estimated to have killed 17 up to 100 million people worldwide.

Source: Our World in Data