[WOIN] Health Advancement

I'm looking over the OLD corebook and I can't for the life of me find any rules about health advancement. I found the part about starting health being a roll of endurance, willpower, and the hardy skill, but there's no indication that characters ever get more. Is health fixed at chargen? Can characters only increase their health in play by taking one of those stats or that skill? Do they just roll another d6 and add it every time they do (I'd assume you wouldn't reroll the whole pool, amusing as that would be)?

Page 54 of OLD - "All derived statistics increase or decrease if their underlying statistics change through character advancement or for other reasons."

So if you increase your WIL or your END or your hardy skill, your HEALTH goes up. If you gain an extra die, simply roll it and add it to your total HEALTH.