He has lungworm, sir. He couldn't report to his post."
This character was only mentioned in dialogue.
Since lungworm is a real species that does not infect Humans, McKenna might not be (completely) Human.
McKenna's first name was revealed to be Kyle in the Star Trek: Starfleet Academy novel The Gemini Agent. The IDW Star Trek: Ongoing comic book Red Level Down revealed that he had a sister at Starfleet Academy named Amy (β), a member of Red Squad, who had also come down with the same condition.