I'm currently reading John Norwich's history of Byzantinum, and he mentions the spread of malaria in Italy and the subsequent population decline as being a factor in Constantine's decision to move the capital east. Why did malaria suddenly become worse around this time? And was malaria a factor in the long term decline of Roman Italy?
I'm reading Mary Beard's SPQR, and she gives a quote from a Roman physician, I think from the time of the Republic, which was something to the effect of "One does not have to read a textbook to study Malaria, it is everywhere around you". The whole idea of the passage was to set up Republican Rome as a dirty place devoid of the stunning architecture which came later. My guess is that it was just a nasty problem which was always there...?