One reason I've heard put forward as to why the Islamic conquest of Byzantium happened so quickly was that the Byzantines were hit very hard by the Justinian plague, but the arid geography of arabia made it harder to spread there, so they were less devastated, is there anything to this?
Yes, it seems likely that they were less affected - but few records exist, so we can't know for sure.. This is likely in any case, because the population density in the Arab world was lower, and bubonic plague most heavily affected larger cities.
From a journal paper I wrote last year, "No records of bubonic plague predate the 541AD Plague of Justinian, and though it is unclear when it first emerged, it may be an example of initial pandemic emergence within the historical period. In that case, it struck Constantinople several centuries later, following a similar population boom, lending credence to the idea that population concentrations are a critical enabling factor." (PDF of my paper: https://www.liebertpub.com/doi/pdf/10.1089/hs.2018.0039 )