Hi
Role name: Plague Bringer
Description: You are a mad scientist who want to cleans all the town by the plague.
Alignment: Neutral Evil/Killing

Abilities: Infect your target at night, the target will die the next night.
The people the target visits, while infected, will become infected.
Doctor can heal and don't spread the plague.
Plague Bringer is immune to the plague.
Witch Doctor convert her target if she is infected.

Win Condition: Be the last person left alive.

Sheriff: Plague Bringer if the Sheriff option is On ; else Not Suspicious.)

Investigator: You discover a number of sharp tools and implements, suggesting that your target is either a Doctor or a Witch, or if you're unlucky, a Witch Doctor or a Plague Bringer.

Host Options:
1) Ignores immunity - (Default: On)
2) Ignores healing - (Default: Off)
3) Doctor who target the Plague Bringer role-block him - (Default: On)
4) Targets are informed that they are infected - (Default: Off)
5) 1 night between infection - (Default: On)
6) 2 night between infection - (Default: Off)
7) Invulnerable at night - (Default: On)
8 ) Infect Role-blocker - (Default: On)
9) Win ties over Arsonist - (Default: Off)

Pros:
- Balanced in my opinion. He is slower than every other Neutral Killing role if he targets a role that can not visits and Doctor got two nights to heal an infected target, but if the Plague spreads the town can die really fast, but can still be stoped or slowed if Doctor heal dead infected targets' targets.
- Doctor is the counter of Plague Bringer which is Role-Play :P

Cons:
- Maybe make the Doctor too strong

Overall effect on the metagame if implemented:
- Make Doctor and Witch Doctor more entertaining to play.
- A fun mixture of Arsonist and Mass Murderer.

Additional comments:
- A infected Mafia member or a
eEvildoer even if he dies can intentionally visit a town member to infect him and hope to be healed.
- A infected Town member will put his target in his last will, so the Doctor can heal the Plague from them.
- Doctor have to be very carefull if someone die from plague, and try to stop or slow it as soon as possible.
- Plague Bringer should infect role that can visit, or he will be slower than other Neutral Killing but he is faster if the Plague spreads.