Weak Doctor is a somewhat common variation of the Doctor role. The Weak Doctor protects its patient from a single kill at Night, but if it targets scum with this protection, the Weak Doctor will also die that Night.
This role became popular enough that Weak became used as a standalone modifier.
The Weak Doctor dies if it targets scum, which is a vague term. In games with third-party roles, the moderator should note before the game exactly who the Weak Doctor should be able to target without dying.
Use and Power
The real power of a Weak Doctor is not its protection. Its use is in confirming innocents like a Cop - if the Weak Doctor targets someone and is still alive the next Day, barring outside interference this implies that the Weak Doctor's patient was Town. Thus, it should be balanced like a Cop.
Note that the Weak Doctor's suicide should be counted when considering possible worst-case scenarios in regards to how many players can be alive.
Weak Doctors are virtually always aligned with the Town.
Weak Doctor should avoid directly claiming where possible. As there are usually not any other protective roles in the game, claiming Weak Doctor is essentially an invitation to eat the next Night-kill so as to prevent the Weak Doctor from reliably determining the alignment of the next person it protects.
With that in mind, the safest way to play Weak Doctor is to try to confirm as many innocents with it as possible. However, given that scum are known for appearing pro-Town, this is not completely reliable. Other players will try to deliberately commit suicide by protecting possible scum with it. Either way, the Weak Doctor should make it a point to clearly breadcrumb its previous targets in the event of its death. It is easy to tell when a Weak Doctor protected scum, because the Weak Doctor dies as an extra kill. It's more difficult to identify who the scum the Weak Doctor targeted was without some kind of clear direction.