Catch It If You Can!

Walkthrough

Get a rare or remarkable status ailment: Mute, Bane, or Plague. These are added effect from hits from specific mobs. The normal status ailments Silence, Curse, or Disease will not work.
Many of these effects will not last long, so it is best to set your home point at HP #1 in Port Windurst, which is very close to Ohruru, and carry a scroll of Instant Warp to get there quickly once you are afflicted.
Mute is given by Eight of Cups in the Outer Horutoto Ruins - Dahlia Tower (F-11) in West Sarutabaruta.

Eight of Cups inflicts Mute through an Additional Effect by its weapon. This will have a difficult time inflicting a high level player.
Mute in Beadeaux from "The Mute" does not work as it is actually Silence.

Bane is given by Lost Soul mages in the Eldieme Necropolis.
Buffalo in Uleguerand Range can give you Plague, however its time affecting the player is limited (see talk page).
Promathia can also inflict Plague. You can fight him on a lower difficulty of the high tier mission battlefield Dawn. It'll cost you 10 merit points per try, and you have to be careful which trusts you summon. Don't summon offensive trusts (they will kill him too quickly), and many of the healer trusts will cure you of Plague as soon as you catch it. Best choice is Koru-Moru, who will cure but not heal status ailments. Other RDM trusts should work as well.
Deathmaw Orobon in Ceizak Battlegrounds can inflict Plague on a high level player with Vile Belch, however, the only way to get this quest done using them is to set your Home Point to the crystal directly outside of Ohruru's location and warp as soon as it is inflicted. It does not last long, but it's one of the few options for a Level 99.
Simply speak with Ohruru when you're afflicted with any of the status ailments.
She will give you 1,000gil for Mute, 1,200gil for Bane, or 1,500gil for Plague. She will also heal the status ailment.