Patricia Berne: It's a play on words of course, referring to the fact that people with disabilities have been called invalids pronounced in-VUH-lid.
Three or four days passed away; the invalids were all well again; the usual habits of the house were resumed, various guests arrived, and
Among the invalids was the daughter of this duke; she was not only suffering from the same complaint, but she was so like
I then called for my medicine-chest, and having weighed several three-grain doses of tartar emetic, I called the invalids, and insisted upon their taking the medicine before they started, or they might become seriously ill upon the road, which for three days 'march was uninhabited.
As you say, there are very few 'invalids' who are too badly disabled to work.