I like how the Malaise's impact on Tobyn's senses and the fact that Keller had been raised outside of the shifter world usurped the mate-scent convention. In the typical shifter romance, at least one of the characters would have some awareness of the other as his mate, but you've set up a nice, slow build over the whirlwind affairs that dominate shifter romances. Since shifter immune systems make them, well, immune to human disease, would the Malaise be a shifter immunity resistant superbug, some kind of cancer (like in MetaWolf), or maybe an auto-immune disease? Something that would be interesting would be the possibility of genetic disorders. Shifter packs tend to be insular and isolated, and there are strong prohibitions against turning outsiders. Yet, I've never seen the issue of inbreeding, nor the resulting genetic problems, addressed in shifter fiction. Perhaps the ability to turn humans (or mate with them, at least), or the instinct to search beyond one's pack for a true mate, was born out of the need for genetic diversity within shifter clans. If the Malaise was a genetic disorder, then the decision to further isolate themselves, and to suspend mate quests, would only compound the effects.