I recently learned that malaria became endemic in Rome somewhere in the 4th century, until the swamps were drained in modern times, and that this lead to northern rulers such as the Holy Roman Emperors avoiding the city in summer and never stationing (northern) troops there. Since Rome was such a powerful symbol, I would imagine ruling from Rome would have been a hotly contested position otherwise but I always only learned about the better defensiveness of Ravenna, and that the late emperor's and Ostrogoths therefore ruled from there. Was malaria a (substantial) factor in this? Thanks!
While there is always more room to be discussed further, I wrote a post to the similar post as OP before in: Is there a link between Malaria and the Popes prevailing in the Investiture Controversy.
In short, while the proposed hypothesis can be at least partly valid, it unfortunately is very difficult to evaluate 'how big' part from a point of view of the current academic consensus, since there were many more factors that external rulers tend to avoid the constant stay and rule in Rome, such as the incessant power struggle among the local aristocrats and the other inhabitants later there.