This is one heck of a heart-warming story. Don't say you haven't been warned!
The 5 year old in question is called Brody and he's currently recovering a 9 hour liver transplant operation. How does this relate to gaming? Well, Brody is a big fan of Halo. His dad, who is a Redditor, posted about the operation on Reddit here. Long story short, the folks at Bungie heard about Brody's operation and subsequent difficulties during recovery and decided to try and help out in some way.
Christine, from the Bungie, visited Brody and his parents in hospital and treated him to some pretty awesome Halo-themed gifts. Christine brought with her a card signed by the Bungie team, a Halo t-shirt, and a life-sized replica of Carter-A259's bright blue helmet, from Halo: Reach. I've pasted a few of the pictures below, you can find the rest here.
It seems it had the desired effect too. Brody's dad later commented: "My family can't thank Bungie enough. I know that they have had a huge impact on how well my son has been recovering after his surgery. My son was a different person after her visit."