During the Cretaceous, 90% of the land surface was covered by oceans; this was a time of extensive epeiric seas. At the end of the Cretaceous, sea levels fell drastically. The drop in sea level opened up land bridges (such as the Bering Strait) between continents.
The opening of land bridges allowed larger animals to cross and mingle with other organisms across continents. As each species had largely evolved isolated from the other, they had become immune to the local parasites and diseases. As the mixing between the organisms took place, foreign diseases, which the organisms were not immune to, were introduced to them. The migratory population succumbed to the diseases of the foreign land. The theory states that only large animals made the trek which explains their extinction in larger numbers. Smaller animals were incapable of such long migration so they did not venture out. Marine species were exterminated because their favourite habitat - the shallow seas - dried up.
Pros for Epidemic Theory:
no real pros for this theory.
Cons for Epidemic Theory
does not account for plate tectonics. The Pacific and the Atlantic (which was newly forming) were too wide for any land bridges to have formed simply because of sea level drop.
notion of smaller organisms not being able to migrate is incorrect. They simply take longer to migrate.
marine organisms could simply have not died out because of habitat. There are approximately the same number of shallow marine environments today as they were at the end of the Cretaceous.
disease alone cannot cause the extermination of whole species, let alone several lineages of a genera.
does not explain the climatic changes that were observed in the Cretaceous.