Sincyllic Fever is a disease that existed during the Cretaceous Period; this is the disease responsible for the death of Skye's father. Anyone that is chosen to journey to Terra Nova must receive inoculations for Sincyllic Fever. The Sixers are the only ones who have medicine for it.
Skye's mother resides in the Sixers' infirmary and the Sixers supposedly will treat her in exchange for Skye giving them information. ("Now You See Me")