What are the Doctors? from The Dark Tower wikia page:
The Cam Tam, also known as Doctor Bugs or Doctors of Eluria, are
insects about the size of a small honeybee and are used by the Little
Sisters of Eluria. Unlike the Grandfather Fleas they are healers that
feed off disease and fix broken bones; while they are healing they
sing music that is very entrancing. Because of this these little
creatures help the Little Sisters blend in as nurses.
So in short, the Doctors feed on disease and that is what they are getting in return. They eat the disease and the people are healed as a result.
Why did Sister Jenna turn into the bugs?
She was damned by the Little Sisters and, as such, her body dissolved into Cam Tam.