Zach Olson

 was a member of a US Navy Submarine that was attacked in the Indian Ocean by the Praxeus infection

After being infected himself he washed up in Madagascar and was pulled to safety by the Doctor, Aramu and Suki Cheng.

He survived just long enough to tell the Doctor about the fate of his submarine and warn her about Praxeus before the infection overwhelmed his body and he exploded. (TV: Praxeus)