The plural form of the noun 'infirmity' is infirmities.
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The word 'infirmities' is found 12 times in the King James Version (KJV). Each time it appears in the New Testament two of which are found in the gospels (Mt 8:18, Lk 5:15).
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The plural of infirmity is infirmities. For most words ending in
y, change it to i and add es.