Then, what was Nagisa's illness?
The reasons are that Nagisa died not because of it directly, but because of a fever that became deadly due to Nagisa's weakened immunity and it transmitted to Ushio during her birth and later made her too weak to withstand fever as well.”
Similarly, is Fuko Ibuki a ghost? Yes. Technically, Fuko was only a spirit wondering the school while her body was in a coma.. I'm honestly not quite sure why some people could and could not see her.. ^^; But yes, she was technically a "ghost".
Simply so, why does Nagisa get sick?
Nagisa becomes pregnant in A New Family, and she and Tomoya decide to name their child Ushio Okazaki, borrowing the root of Nagisa's name. In White Darkness, Nagisa enters her final months of pregnancy, and the strain on her body causes her to fall ill once more.
What happened to Ushio?
In the coming winter, being filled with the desperation of wanting to do anything for Ushio, Tomoya brings her for a trip, but she falls unconscious and dies shortly after along with her father who succumbed to his grief. Ushio is then revealed to be the Girl in the Illusionary World.
