"But the evil spirit answered them, 'Jesus I know, and Paul I know; but who are you?'" (Acts 19:15 RSV).
In this case, what was the evil spirit and why was it only familiar with Paul and Jesus? Others could not accomplish driving it away.
The entirety of Chapter 19 concerns Paul's extensive ministry in the prominent city of Ephesus, including not only teachings, but also many miracles that were accomplished "by the hands of Paul." Verses 12-14 tell us that the seven sons of a Jewish high priest named Sceva decided to effect healings themselves by calling upon "the Jesus whom Paul preaches." The result is the confrontation at 19:15.
Since the topic here is healing, I would surmise that the 'evil spirit' was expressing as illness. There is, of course, no physical evil spirit present; spirits do not have tangible form. Clearly, these attempts to co-opt Paul's work were ineffective, because they were not expressions of deeply held faith. Truth is not deceived by insincerity. Healing is not a gimmick that can be evoked by saying the right magic words.