Carrier (3.5e Feat)
From D&D Wiki
You are a carrier of a dangerous disease, though you are immune to its effects.
Prerequisite: Must have one level of a Fiend class.
Benefit: When you gain this ability, choose a disease with a DC equal to the DC your disease would have (10 +½ HD + Con mod). Your disease does ability damage or special effects equal to the disease chosen. Once chosen, your disease type does not change, but your disease DC will increase when your HD or Con modifier increase. Unlike a normal disease, this is a supernatural disease, and its initial effects occur immediately.