Writer's tool kit
Take some form of ill health and look at the ordinary vocabulary we have for talking about it, and you'll find some peculiarities.
Why bother? Good question - we'll come back to that at the end.
In English, you can have "a cold", but not "a flu". On the other hand, you can have "the flu" (or just plain "flu"), but you can't have "the pneumonia". In grammatical terms, colds are countable, flu may be definite, but pneumonia is always indefinite and uncountable.
Most diseases are like pneumonia, so colds and flu are the odd ones out. A similar countable illness is a chill. And another very definite one from the past was the plague.
Then there are aches and pains. You can "have a headache", but you'll never "have headache" without the benefit of an article - unlike toothache, back-ache and all the other aches you can think of. Most aches are vague, uncountable states, like misery; but for some reason headaches are single countable experiences.
These are all medical nouns. Adjectives seem no more rational. You may be "ill", but you're not usually "an ill person" - though you may be suffering "ill health" or the "ill effects" of your life style. Nor can you usually be described as a "well person". These adjectives are fine after be ("She is ill"), but not much use as modifiers for person nouns ("an ill child").
Of course, if you want to talk about unwell children you can always use "sick". But even here you have to watch out for unwanted side-effects of the grammatical cure. Think of the different medical conditions described by these sentences:
"Oh, she's sick today so she's stayed at home."
After eating three ice-creams, he was sick all over the place.
Whenever I go on planes I feel sick.
But the good news is that a sick child only has one of those conditions.
We could go on. But what's all this got to do with the writer's tool kit? The point is that we all intuitively know these things - a vast amount of detail about particular words, not all of which makes sense.
A large vocabulary doesn't only contain a lot of words, it also contains a lot of information about each individual word.
The occasion of classes depleted by illness might just lend itself to a brief exploration of the language of sickness. Pupils could explore the quaint intricacies of words and images that we so often take for granted.
But be warned: it could leave you sick as a parrot - though certainly not sick of grammar.
Richard Hudson is professor of linguistics at University College, London Geoff Barton is headteacher of King Edward VI School, Bury St Edmunds, Suffolk