Complex sentence: In 2013--2014, there was an outbreak of Zika fever in French Polynesia that was the largest outbreak of Zika fever before the outbreak in the Americas that began in 2015.
Split into simpler sentences: In 2013--2014, there was an outbreak of Zika fever in French Polynesia. The outbreak in French Polynesia was the largest outbreak of Zika fever before the outbreak in the Americas that began in 2015.