Red berries would be left over from last fall, Pennyfarthing, so they are probably just rotting away naturally. I'm surprised that they have lasted this long, anyway. A maggot may just be part of the rotting process, or it may actually be a larva of a completely different insect species. The larvae of certain kinds of moths, sawflies, or beetles can be hard to tell apart from maggots.
This year's crop of berries is probably still green, or just barely starting to turn color.

sounds like they are just dying back naturally. But have they been on for a long while.

It's very odd. To be honest I can't remember when they formed, but they have been going red all summer. If they really are from last year, then there won't be any this year, as there aren't any more.

Alternate bearing isn't unknown among Hollies, Pennyfarthing, especially if the birds don't eat the previous year's crop. Do you remember if there were any tiny white flowers in among the leaves this spring/early summer? Also the little green berries will be pretty stealthy until they fatten up and turn red this fall--they're easy to miss.

Thanks Tug. Yes, there were a few white flowers so I guess I'll just watch for berries. I'm relieved to think the holly is not diseased and might do better in the future.