It's a cruel world out there
Yes, it is. A recent study suggests that healthy lobsters avoid sick ones like . . . well, like the plague. This is a very effective means of infection control, of course, so we can't blame the crusty old beasts.
I could make some comments about how humans do this too--how modern medicine allows us to shunt the sick off into some other place, where we only have to see them at our leisure--but I won't.
Because, in fact, I think the important part is that it is, to use Thomas's words for an idea with which he may not agree, proper to humans to care for the sick, even at their own peril. It is morally praiseworthy in most societies, and positively obligatory in Christian ones, to risk infection, inconvenience, and intimacy by caring for one whose body is in revolt.
Thomas says that man's proper activity--the activity that distinguishes man from other animals, the activity that is most emblematic of man and therefore whose exercise is most necessary to man's happiness, the activity without which man would not be man--is Reason. Rationality is that activity which man shares with the higher beings (angels, God), and in which the lower ones (animals, even the highest non-human animal) do not participate. It might almost be said to be the imago dei.
Sometimes I buy this, but other times I sense that Thomas was dead wrong.
Sometimes, I think self-sacrificing compassion is the imago dei, man's proper activity, the activity without which man would not be man.