Due to illness.

A generic and feeble excuse that was highly exploited during the pandemic as to why something will not happen within an agreed timescale, maybe will never happen at all or to explain someone's non attendance at a meeting, event or function.

Dave apologises for not being able to attend baby Tarquin's christening today.
Yes it's really disappointing his missed Tarquin's big day has he said why he can't make it?
He just said it was due to illness.