Iosefka's Clinic is one of the two locations where the Hunter can send people looking for "safe places", the other being Oedon Chapel. If the Hunter returns to the 1st Floor Sickroom after defeating Father Gascoigne, the woman behind the door directly behind the lamp ( Iosefka) will claim that she wishes to help people during the night of the hunt, and asks the Hunter to send anyone they should find to her. Anyone the Hunter sends here will be experimented on and transformed.
The Hunter can enter the Clinic through a hidden path in the Forbidden Woods, found by sticking to the left side of the Wood and navigating a cave filled with poison water. After climbing an obscenely long ladder, the Hunter will appear in the graveyard just outside the Sickroom, and can unlock the gate.
Inside the Clinic is a Cainhurst Summons, which is necessary to access the hidden zone Castle Cainhurst and the two covenants Vilebloods and Executioners.
Iosefka herself can be fought and killed here. It is advised to do so only after defeating Rom, the Vacuous Spider, so she will drop a Third of Umbilical Cord.