Simplify the following text for a Grade 7 native English-speaking person.
Original: Influenza , commonly referred to as the flu , is an infectious disease caused by RNA viruses of the family Orthomyxoviridae ( the influenza viruses ) , that affects birds and mammals .
Simplified: It is caused by several different viruses ( see : RNA virus ) , which is why people can have the flu more than once . . The name influenza comes from , meaning `` influence '' .