Why is 'diabetes' plural?

This is a good question! It is because diabetes is a singular Noun, even though it has the "s" at the end. The root of the word is from Greek.

Some singular nouns in English will end with an s, like news, measles, gymnastics...

The s at the end of a word will often mean it is plural, but not always.

Stroke is also a singular noun.

I hope that helps:)

'Diabetes' is not plural; it is uncountable.

It ends with an 's' because it is a loanword form Ancient Greek-which has many words that end in 's'.

For example: pathos, kudos, chaos, mathematics, physics.

Even normal English words can end in 's' without being plural: bus, glass, lens.