Complex sentence: The Asian Flu was a pandemic outbreak of H2N2 avian influenza that originated in China in 1957, spread worldwide that same year during which a flu vaccine was developed, lasted until 1958 and caused between one and four million deaths.
Split into simpler sentences: The Asian Flu was a pandemic outbreak of H2N2 avian influenza that originated in China in 1957 and spread worldwide that same year. This virus pandemic lasted until 1958 and caused between one and four million deaths.