What was wrong with the seamstress's little boy in "The Happy Prince"?

There are a number of hardships displayed in Oscar Wilde's short story "The Happy Prince." The statue of the Happy Prince is initially able to see the sadness contained within his city, and he directs the helpful swallow to take various aspects of the statue's own body to help the people who need it. The seamstress and her little boy are two of those who need help. The statue of the Happy Prince tells the little swallow about the poor woman and her child, saying "[h]er face is thin and worn, and she has coarse, red hands, all pricked by the needle, for she is a seamstress."The statue of the Happy Prince then turns his attention to the child, saying,

In a bed in the corner of the room her little boy is lying ill. He has a fever, and is asking for oranges. His mother has nothing to give him but river water, so he is crying.

The statue asks the swallow to deliver a ruby from the statue's sword so that the mother can afford water and help her son. Initially the swallow is hesitant, but the statue convinces the bird because the boy is so thirsty.When the swallow arrives at the poor house, Wilde writes that "[t]he boy was tossing feverishly on his bed, and the mother had fallen asleep, she was so tired. In [the swallow] hopped, and laid the great ruby on the table beside the woman’s thimble. Then he flew gently round the bed, fanning the boy’s forehead with his wings. “How cool I feel,” said the boy, “I must be getting better”; and he sank into a delicious slumber. While the specific illness of the boy is never specified, it is clear that the child is sick, and some symptoms of the sickness are thirst and a high fever, which could be brought about by dehydration.