Did infectious diseases and even meso-level / meso-time climate change have more of an effect on the disintegration of the Roman Empire than older historians thought with fingering barbarian invasions, the rise of Christianity, etc?
Certainly.
Were they decisive or nearly so?
Likely not.
So, the "No, really?" is rhetorical, in case that weren't already clear.
Was there a "Plague of Cyprian" and was it ebola or something similar, as Harper claims in the book and per that Wiki page?
Almost certainly not.
Per this Cambridge monograph by Sabine Huebner in response to him, Harper is almost certainly wrong on the dates of this plague and its origin, ergo wrong on its source. Independently of all that, the respondent says he's almost certainly wrong on this plague of non-Cyprian causing devastation in hinterlands Egypt. And derived from that, he's even more certainly wrong on claims that it nearly toppled the empire.
Roman historians know this period was the tail end of the famous crisis of the third century. Population loss from infectious diseases were a problem, but whatever this plague was and whenever its dates, it could not have nearly toppled the empire. Even at Harper's early date, this is 15 years after the end of the Severan dynasty and the instability that had already introduced.
And, it flat period could not have led to the Decian order for sacrifices and the Decian persecution, contra Harper. Per Huebner, the first mention of this plague did not occur until after Decius was dead.
With Huebner as co-author, here are parts one, two and three of a multi-academic response to Harper on various issues. (The first is 13 pages; the second and third are 10-pages, all easy reads.) Part two starts by noting his taking an extreme position on the Antonine plague and also flat-out ignoring a lot of modern research. The fact that no major invasions from the east happened in the first years after the Antonine plague and that, in the longer term, after the 192-93 coups, the Severan dynasty ruled 42 years, all undercut Harper. Beyond the Plague of Cyprian, part 1 is an overview critique of the whole book, much of it focused on problems with Harper's "maximalist" take on climate change. Part 3 looks to a fair degree at his take on the Justinian plauge.
And, with that, I'll take a pass on this book. And probably on Harper in general. (IIRC, I saw his "Plagues upon the Earth" at my library about a year ago and took a pass.)
He's also writing a third book in this same general line, about problems with human overgrowth. It, too, may be interesting, but will likely run a narrow maximalist take on modern plagues.
Related to that? Per this person's thesis, Harper reportedly has professional problems related to his stridently pushing his thesis, and other personal issues as well. (Per other information, the intersection of professional and personal were student protests at Oklahoma University alleging that he didn't take seriously enough two cases of faculty reportedly using the N-word, and of blackface on campus, and that also questioned his dedication to diversity, equity and inclusion. This was when he was OU provost. When the OU student newspaper's header uses the "long-embattled" cliche, you've got problems.) The graduate also notes that Harper, and Amber Kearns, who has followed in his footsteps in some ways, also ignore the possible polemical value of writings from that general time about the plague's virulence, including overstating its death rate and its effect on the empire. And, on Harper's case, if conservative evangelical White Christianity accompanies the background to the student protests, I can see the background to him NOT discounting the polemical value of Christian letters overstating the death rate and calling it judgment on the empire. OTOH, lower-star reviews of his "Plagues upon the Earth" dinged him for beating evolution like a dead horse, as in people who are NOT fundagelicals were saying "we know evolution, you're beating a dead horse."
As for what this plague might have been? The Plague of Justinian has now been fairly well identified as the first incursion of the bubonic plague. Could this have been an earlier version? The "thesis" link suggests Lassa fever, which I also find unlikely, if nothing else as with Ebola on the grounds of where the plague originated, and tentatively suggests measles, which I could certainly accept.