Source: Mythic Adventures, pg(s). 166
Plaguebringer can try to infect a new host by entering its body; if it fails, Plaguebringer cannot retry infecting the same host for a month. The host of Plaguebringer will contract any disease exposed to them, but also suffers no ill effects from them, essentially turning them into an asymptomatic carrier and spreader for all the diseases of the world. To aid the host in this, Plaguebringer grants an array of abilities ranging from an aura of unlife that heals undead and harms the living, to a vampiric touch capability, to the ability to summon a nightwing.
When infecting a host, Plaguebringer has full access to its host's senses along with darkvision and blindsense with a range of 120 feet. Hosts that please it are protected by Plaguebringer; however, if they resist it, Plaguebringer will withhold its abilities and seek to take control and then leave, forcing the former host to suffer the symptoms of all the diseases they previously contracted.