The flu virus is a geometric sequence
Each person infects two more people with the flu virus, making the number of recently-infected people the nth term in a geometric sequence.
Source
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"Free High School Science Texts Project, Sequences and Series: Arithmetic & Geometric Sequences, Recursive Formulae (Grade 12). December 10, 2012."
http://cnx.org/content/m39302/latest/#uid16
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