JLA #42A

Half a Mind To Save a World
While assisting with a delicate neurosurgery, the Atom discovers that the unusual growth in a boy's brain is actually a bacterial civilization, and that the scheduled surgery would slaughter the intelligent life present while curing the boy. He calls upon the JLA to help negotiate a means to prevent this, but when their powers fail at that size, they have to improvise in a hostile environment.
Release Date: 2000