illness/illnesses

[Tan Elaine]

If my son and daughter are ill and I pray for their quick recovery, which of the following word in bold should I use?

I pray that my son's and daughter's illness/illnesses be cured soon.

Thanks.

[birdeen's call]

Do they have the same illness or different illnesses?

[Tan Elaine]

Do they have the same illness or different illnesses?

Thanks, Birdeen. They have the same illness.

[birdeen's call]

Then there doesn't seem to be a reason to use the plural.

If they had different illnesses, both the plural and the singular form would be correct in my opinion, with slightly different meanings.

[5jj]

I pray that my son's and daughter's illness/illnesses be cured soon.

More natural for most speakers of BrE would be:

I pray that my son and daughter's illness is/will be cured.
We probably would not put the apostrophe s on son, and very few people would use the subjunctive be.