A new virus emerges and spreads like wildfire. In order to contain it, researchers must first collect data about who's been infected. Two main viral testing techniques are critical: one tells you if you have the virus and the other shows if you've already had it. So, how exactly do these tests work? Cella Wright explores the science of PCR tests and immunoassays. [Directed by Hype CG, narrated by Bethany Cutmore-Scott, music by Gabriel Maia].