The most infamous pandemic was "Spanish Flu" which affected large parts of world occured in which year?

1918

The principal reasons for the decline in the death rate after 1921 were increased levels of control over famines and epidemic diseases. The latter cause was perhaps the most important. The major epidemic diseases in the past were fevers of various sorts, plague, smallpox and cholera. But the single biggest epidemic was the influenza epidemic of 1918-19, which killed as many as 170 lakh people, or about 5% of the total population of India at that time.  Also known as ‘Spanish Flu’, the influenza pandemic was a global phenomenon.