"I haven't started on the damaged tissue yet," Drake warned, but then added, "but it shouldn't be much different. I wanted to halt the ongoing damage, first. If it comes back -- you are more prone, after all -- just come to me and I'll purge it out again. I just finished with that."

It really wasn't that big of a deal to play temporary immune system.

Now, for the older damage.

He'd picked his spot, somewhere where Cassian would get the most immediate benefit from having it fixed. Neighboring against healthy tissue, old scarring was only going to restrict that healthy bit of lung from doing what it could do.

Bit by bit, he started unraveling and re-purposing old scarred tissue, growing new lung in its place. Done in such tiny increments that the body would have naturally tried to grow it back, like muscle fibers repairing over time, but sped up a thousand fold by Drake's energy and guided by his Gift.