I wonder what his illness was. I know people were sick all the time in those days from something or other ...
If a letter or diary mentions that a woman “was sick,” it usually means she was giving birth.
According to one source I found there was a worldwide influenza epidemic in 1857-59. That could lay a bunch of people low without generating publicity and Harper’s Weekly articles like a yellow fever or typhus epidemic would.
http://www.bellaonline.com/articles/art7040.asp