Plague

A specific plague existed in Seraph of the End, one that was able to infect seraphs. In prehistorical times, it seemingly had infected a large proportion of the seraph population. Its existence appeared a major reason for Sika Madu to research forbidden life affecting areas.

Background
During Earth's prehistory there was a seraph civilization established. While homunculus and dinosaurs were other species that existed at the time, the plague apparently only affected seraphs. The existence of the plague appears the reason why Sika Madu created Mikaela as a means to find a cure for it.

Although creating, extending, and reviving life were said to be grave sins, Sika Madu was willing to examine these areas anyway to save those who followed him from this plague. The plague was a factor in mind for Sika Madu who also planned to use the Tower of Babel to get them to paradise as part of the Babel Project. That the plague was killing angels was addressed when Sika Madu shared his plan of extending the lives of angels to God.

Features
Until the effects take hold, an individual infected with this plague can appear in good health with no signs at all of illness. Similarly they are also able to communicate without any apparent difficulties. The only sign of the plague is immediate, and signalled by coughing. A notable quantity of blood spills from the mouth and drips down. Sweating and trembling may be symptoms of the plague, or the shock at what has transpired.

It would a pneumonic plague given it affects the respiratory system. Implied to be incurable, the plague has spread to the extent that it may affect up to half of a seraph population. The plague seems to factor in having an angel's wings blacken. There appear to be stages to the plague as it was said to have advanced when Paimon felt near death.

Prehistory
When Paimon is speaking to Sika Madu regarding having Mikaela killed it becomes apparent that he is unwell. Suddenly coughing, after blood trickles down Paimon's mouth, Sika Madu addresses how he is infected too. When Sika Madu asks why Paimon hid it, he does not reply. Instead, Paimon questions whether Mikaela was made as an experiment to find a cure for the plague endemic to their kind.

Responding that something must be done, Sika Madu was wary that even if they do reach paradise over half the angels that follow him will die. Although ill, Paimon was still able to argue his case for Sika Madu to focus on the kingdom overall rather than the ill members in it for the good of getting some to paradise.

Ongoing symptoms of the plague such as coughing were apparent when Paimon entered Mikaela's room where Yu the homunculus also was. Symptoms of the plague may have compounded Paimon's condition as he attempted to murder Mikaela. If the illness were to be cured, that might draw God's punishment to Sika Madu which is what Paimon wanted to avoid.

The illness is said to have advanced by Paimon after he had been defeated. Pointing out what he called his ugly black wings, Paimon felt he was doomed to die anyway. That his sickness would be a concern for his father also factored into Mikaela's responses. Neither Paimon or Mikaela would succumb to the plague as in the circumstances they took their own lives instead

When Bael joined Sika Madu in the tower after Paimon and Mikaela has passed away it would be revealed that he has been infected. That the disease had resulted in the deaths of over half the angels already was a factor in Sika Madu's despair over the losses once Mikaela had perished. Similar to Paimon and Mikaela, the plague infected Bael but would not directly result in his passing as he too would take his own life.

At some point Belial had passed away which was very likely to have been from the plague. There appeared to be a vast valley filled with cross-shaped grave markers in an indication of the numbers of angels that the plague had effected. Where Marbas and Orobas where not seen once Sika Madu announced to others that they were returning to paradise, it is possible they had succumbed to the plague at some unknown point.

After a millennia of burning once a humanoid angel and trumpet wielding seraphs had destroyed the seraph civilization nand all in it, Sika Madu would transport the souls of angels into human bodies that Yu the homunculus had produced. It is not known whether the plague would still affect the fallen seraphs' souls that were in their human host bodies.

Taboo
Where postponing death (of which curing a plague would have resulted in that) was one of the three gravest sins, the plague factored into Sika Madu's attempt to save others anyway. While in mind, the plague and the taboo to break in order to cure it was the third priority. Sika Madu first meant to resurrect others, using the created form of life that Mikaela was. Curing the plague while important, was the third taboo to break after the first two.