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The disease that Kenshin and Kaoru died of in SSH is a symbol of Kenshin's lost swordsman spirit. The soul of a samurai (I know he's not a samurai, but he lives as one) is in his sword. Therefore, when Kenshin passed the Sakabatou to Yahiko, he in effect, gave up his spirit.
This left Kenshin completely vulnerable to all the guilt he'd carried inside him for the past 20 years, but had been able to supress by using the Sakabatou to help people and protect the weak. Now the guilt which had manifested itself only through the unhealed scar consumed him from the inside out until it killed him.
Kenshin's passing the illness on to Kaoru is a symbol of the close bond they had developed. Although Kaoru (in the OVA time line!) could never be as important to Kenshin as Tomoe, she could still have her own place in his life by bearing his burden. This was her right as his wife, which is in keeping with Asian culture where widows are expected to throw themselves on their husbands' funeral pyres.