Nanites
Nanos are microscopic machines that work inside the body. The original nanites worked with the dice codes only to produce the various effects of levelling up, but more recently they have been injecting all Master Dice with ones that do accelerated healing as well to prevent suicide before earning profits.
Painkillers won't work, and as a matter of fact, they do BAD things, like anesthetics and tranquilizers. Anything that's supposed to dull the pain or knock them out *Other than that powder that Stealth suggested, since it's made up of a certain chemical designed to tell the nanites to let Lexx sleep.*
Painkillers cause the nanites to become confused as to what they're supposed to be fixing, anesthetics shut them down completely, which then is a bad thing for since they won't heal normally,.. and tranquilizers,... since most are designed to be immediate cause the nanites to become even more confused and wreck havok on the body. (Basically, they're allergic to these things, even an aspirin will cause more pain than good.)
Fortunately, it is against game rules for anyone to purposely use such things in battle.
The nanos keep a perfect genetic balance. Everything inside works as if they were perfect. If they survive, despite the punishment they may take physically, they will keep them in top physical condition. The nanos don't seem to care much about fixing things on the outside though, which is why they carry scars. They are hasty. The skin is only an enclosure and barrier, that is how they treat it. They will never be naturally sick, they'll never develop certain diseases. They can't contract AIDs, can't get Cancer, can't get anything involving white blood cells as a source of a problem. They will still grow old and eventually their bodies will wear out, ... but their minds would probably wear out before that. If they ever truly 'give up' then the body would go as well.
The nanites take the place of the white blood cells and 'do' away with the immune system because they see it as unnecessary when they fulfill that function. If the nanites were removed, they would have zero immunity and would essentially be a hemophiliac without an immune system at all. They replaced all of it.
