Fringe/Recap/S02 E14

Season 2, Episode 14:

The Bishop Revival
A German scientist develops a virus capable of only targeting and killing people with specific genetic traits. When the Fringe team investigates, Walter realises that the killer is using scientific concepts and formulas developed by his own father, Dr. Robert Bischoff, a German scientist who passed information to the Allies during WWII before immigrating to America. The Fringe team find the scientist's lab just as he's left to plant virus bombs at a conference for world tolerance: they manage to find the bombs but can't locate the scientist. Walter, driven to tranquil fury by the scientist's use of his father's theories, creates a new strain on the virus that only targets the scientist himself and releases it at the conference, killing him. As he dies, the scientist points to Walter and calls him a traitor. At the end of the episode, one of Walter's old photos of his father reveals that the scientist was a Nazi who has somehow lived to be over 100 years old without ageing.