nouns - Is the term 'Invalid' applicable for human beings?

As I was reading a novel 'What Katy Did', I came across an interesting mention of the word Invalid.

His wife was said to be an invalid, and people, when they spoke of him, shook their heads and wondered how the poor woman got on all alone in the house, while her husband was absent.

For me, it sounds off. Can invalid be a valid word for human beings?

I have heard about things like - the credit card is invalid, the transaction is invalid, your registration is invalid..., but how about your husband is invalid?

Answer

invalid there is a noun.

invalid (n) - Someone who is incapacitated by a chronic illness or injury.

Having this said, his wife seems to be very sick, in a crucial condition that might have made her incapacitated.

Now since there's discussion about the degree of being incapacitated (which makes you ultimately invalid), I'm adding a bit to improve this answer.

Here is another reference from OLD:

invalid (n) - a person who needs other people to take care of them, because of illness that they have had for a long time.

Now, if you look at both the definitions, you see that the term invalid ranges from someone being assisted by others to walk, eat or do routine activity to someone who is permanently bedridden (as in the last stage of cancers). Contrary to what Doc and FumbleFingers, it is not always necessary that invalid person is so so so sick that he/she is on the deathbed. And, I'm a doctor and have come across many such patients with chronic illness (in fact, have worked in hospitals that only take such cases).

The OLD further explains it in its example:

She had been a delicate child and her parents had treated her as an invalid

Furthermore, delicate here means:

delicate (n) - (of a person) not strong and easily becoming sick

That's where the WordWeb definition fits in. Invalid is someone who is incapacitated - not able to perform their tasks because of illness that has brought weakness. Here, the child does not necessary to have Ryley's tube or Folly's catheter as Doc mentions.

On the other hand, invalid does not always mean that the person is just incapable to do things and is not so critical. That's why I said, the term applies to incapacitation and this varies from degree to degree depending on the illness that person has.

Check this here:

If you see Saturnino Soncko (a person working in the silver mines of Cerro Rico), he's certainly invalid but I can still argue and deny calling him invalid as at least he is not that incapacitated! In that picture at least he is sitting without any assistance whereas invalid requires support even for this, don't they? They certainly do I see the woman every day. She is an invalid and cannot move anything other than her eyes.

Again, invalid is certainly a serious condition but it varies in degrees or severity depending upon the type of illness. I'm not sure to apply partially invalid or completely invalid for that though it might make better sense.