What mystifies me is, that he doesn't seem to have had anything particular the matter with him.
There he lies, getting worse and worse every day, without any specific ailment.

It's a strange illness, Philip." "I don't see anything strange in it." "Don't you?
Don't you think the surrounding circumstances are strange?
Here is this man comes to your house hale and hearty; and all of a sudden he falls ill, and gets lower and lower every day, without anybody being able to say why or wherefore." "That's not true, George.

Everybody in this house knows the cause of Tom Halliday's illness.

He came home in wet clothes, and insisted on keeping them on.

He caught a cold; which resulted in low fever.