Continuing the look at synonyms through the eyes of the etymologist, this time it is the letter I and ill. Interestingly the word 'ill', while not considered to be related to 'evil', was used in Middle English to mean 'offensive, objectionable, malevolent, unfortunate, hurtful'. It seems to have come from Old Norse, where illr meant 'evil, bad' and even 'mean, stingy'. Not until the late 14th century do we see it used to mean 'unwell'.
Unwell is clearly a negative form of 'well', itself a Germanic word which has mostly been used to mean 'abundantly, very much', and only used in the health sense from around 1550. The word is derived from Proto-Indo-European wel meaning 'to wish'.
Sick is from Proto-Germanic seuka meaning the same as it does today. It has only been used in the sense of nausea since 1610.
Ail has more often been used to mean 'loathsome, troublesome' and can be traced back to Proto-Indo-European agh-lo 'to be depressed, afraid'.
Poorly has only been used in the sense of ill health since the middle of the 18th century. I clearly is derived from 'poor', and thus from Proto-Indo-European pau 'few, little'.
Infirm comes from the Latin infirmus or 'weak, frail, feeble' and, in turn, from Proto-Indo-European dher 'to hold firmly, support'.