Try instead the disease zombie plague
To challenge the frame, I suggest you roll instead with published material. The description of the 4th-level Sor/Wiz spell animate infectious zombie [necro] (City of Stormreach 144) includes this disease:
Zombie Plague: The disease carried by an infectious zombie has an incubation period of 24 hours. After that time, a victim who failed its Fortitude save loses 1d6 points of Constitution. An additional save must be attempted every day thereafter until the victim either succeeds, negating the plague, or dies, rising as an infectious zombie.
An injury from a zombie's natural weapon spreads the disease; it has a Fort save DC of 16.
The game codifies diseases and provides ways to deal with diseases that are both mundane (like the Heal skill's treat disease and many feats1) and magical (like the spell remove disease). The save DC is high enough that commoners frequently fail and unprepared low-level PC worry but not so high that it's impossible to deal with. (Your sample effect, for example, has otherwise CR 6 creatures with a special ability that has a save DC of 30.)
About this special ability
A special ability that kills days after the creature's encountered shouldn't increase a creature's CR significantly unless the effect is very difficult to delay or remove. Such an effect can still kill folks, but treasure gained from such encounters should permit them to get help to end the condition before it kills them. However, the shorter the interval, the more deadly the effect, and the more you should look at increasing a creature's CR because of it.
For example, the onset time of the mummy's the mummy rot is 1 minute. If unprepared and infected, there's not time enough to return to town for a remove curse followed by remove disease—and dying from it means turning to dust and being blown away, so no raise dead and maybe no resurrection either. That's scary.2 But rarely are stories told of adventurers bitten by dire rats who then die after contracting filth fever. With its onset time of 1d3 days, filth fever is just annoying, and dire rats aren't significantly more dangerous because of it.3
Without any way to remove your example's effect, after a few days, an adventurer who fails the save will be a zombie. That makes the only "cure" succumbing to the effect, becoming a zombie, having someone kill you, then having someone cast on you raise dead. That's probably worth a +1 to CR because the effect will kill a few PCs but, afterward, the party, knowing what they're up against, pays strict attention to proper adjudication of encounter distances and Dumpster-dives for ranged weapons that deal slashing damage, and the effect will only come up rarely.
1 In addition to the many feats that grant bonuses to saving throws against disease, there're feats like Deformity (Parasite) (Elder Evils 13) and, obviously, Disease Immunity (Heroes of Horror 121-2). There are also feats that are more interesting in a zombie plague-ridden setting but less common otherwise, like Invoke the Cup and Talisman (Dungeon #104 107), Planar Touchstone (Empyrea Mere (Planar Handbook 155)) (Magic of Incarnum 207), and Vathrin Stigmata (Dragon #319 61).
2 I kid you not. The description of mummy rot says that a creature that dies from it "shrivels away into sand and dust that blow away into nothing at the first wind" (MM 191). Fanciful? Yes. Terrifying? O, yes.
3 Well, to adventurers, anyway. To low-level commoners, dire rats remain a scourge.