From the year 438 to ~950 CE, Oceanyka was ravaged by a series of epidemics. The causes of these disasters were both natural and man-made; the construction of enormous granaries (where rats festered), constant warfare, hygienic decline and superstition helped bring about numerous waves of mass disease.
- Firstly, from 438 to 445 CE, the First Great Pestilence was caused by smallpox. Oceanyka's population had partially recovered from the Ferozen Invasion, so the impact of this epidemic was significant. Over 20% of Oceanyka's population perished.
- Second, from 518 to 540 CE, the Second Great Pestilence occurred, caused by a mass outbreak of Devil Rabies. This period is also known as the First Corpse War, since the symptoms of devil rabies made its victims appear as hyperaggressive living corpses. About 40% of Oceanyka's population perished; most of it died from the violence and famine which emerged from these events, rather than the disease itself.
- Thirdly, from 685 to 689 CE, the Third Great Pestilence caused about 50% of the Oceanykan population to perish from plague, an offshoot of the Justinian Plague which ravaged Europe just a century earlier. It was likely brought by Austronesian maritime traders and did much to strengthen Oceanykan xenophobia.
- Fourth, from 801 to 832 CE, the Fourth Great Pestilence was brought upon by a mutant (and as of now extinct) strain of Devil Rabies, capable of being transmitted by airborne particles released through the decay of corpses. This period, the Second Corpse War, was largely believed to be an apocalyptic event back then. About 20% of Oceanyka's population perished; rather than division and witch-hunting as the previous outbreak of this disease had caused, most Oceanykan polities found common ground in their fight against the living dead. Corpses were disposed of by mass cremation, with both the bureaucracy and religious institutions growing to face this threat.
- Finally, from 938 to 950 CE, the Fifth Great Pestilence killed about 20% of Oceanyka's population, caused by an outbreak of plague. This reduced mortality was achieved through the hard-earned knowledge of the previous epidemics and the actions of more centralised states, which were able to respond more effectively to such a threat.
In the 1400s an offshoot of the Black Death arrived at Oceanyka, but its continental mortality did not surpass the 10% mark, primarily because of the efforts of modern, centralised states with a millennium of knowledge on limiting the spread of pandemic diseases.