Simplify the following text for a Grade 12 native English-speaking person.
Original: At the time of the creation of the World Health Organization ( WHO ) , in 1948 , health was defined as `` a state of complete physical , mental , and social well-being and not merely the absence of disease or infirmity '' .
Simplified: Health is `` a state of complete physical , mental , and social well-being and not merely the absence of disease or infirmity '' , according to the World Health Organization ( WHO ) .