What is pandemic flu?

"Flu that spreads rapidly causing widespread epidemics around the world.
Pandemic influenza occurs when a new, highly infectious and dangerous strain of
the influenza virus appears.
In contrast to the ‘ordinary’ or `seasonal’, flu outbreaks which we see every winter
in the UK, flu pandemics occur infrequently – usually every few decades. There
were three in the last century. The most serious was in 1918, killing millions of
people worldwide. Smaller pandemics happened in 1957 and 1968."