Most of us have Epstein-Barr Virus (EBV) hiding in our bodies.
You may know it as the cause of glandular fever or “kissing disease”.
But more research is suggesting that it can turn our immune systems against us to cause autoimmune disease, such as multiple sclerosis or lupus. So, what is the virus doing in our bodies - and can we do anything about it?
James is joined by Dr Claire Shannon-Lowe, a virologist at the University of Birmingham, and Dr Patrick Kearns from the University of Edinburgh, a neurologist and epidemiologist.
And we delve into the world of echolocation.
Bats hunt at night by making pulses of sounds and listening to the echoes so they can build a picture of the world around them in total darkness - and it's a technique some people with visual impairment use to explore the world.
A team at the University of East Anglia has been testing how well humans can pick up these skills, so James pays a visit to have a go...
Presenter: James Gallagher Producers: Gerry Holt, Tom Bonnett & Thomas Hunt Production coordinator: Stuart Laws Content editor: Ilan Goodman