It’s cause you’re in the BFA version of Undercity, which is covered in plague. Head to Zidormi to switch to the Cata version:
Zidormi - NPC - World of Warcraft (wowhead.com)
And if she’s missing… well, good luck, there’s many of us in that unfortunate club.
Zidormi missing from Tirisfal Glades - Support / Bug Report - World of Warcraft Forums (blizzard.com)
Hallow’s End Lordaeron phasing broken - Support / Bug Report - World of Warcraft Forums (blizzard.com)