adjective
1
BAD : I am being trained to look after ill children.
GOOD : I am being trained to look after sick children.
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Ill (= sick) is usually used after a verb: 'I told the doctor that I felt ill.' 'His father is seriously ill in hospital.'
Before a noun, use sick : 'Your father is a very sick man.'
2
BAD : He wanted to be with his son who was badly ill.
GOOD : He wanted to be with his son who was seriously ill.
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seriously ill (NOT badly ): 'So far three people have died and five more are seriously ill.'