Whole chicken flock dead within a week - what could it be?

We had four chickens - three of them were about four years old and one we'd hatched last August. A week ago, one of the older chickens was dead one morning. We put it down to old age and her just being a bit of a feeble chicken anyway. We went on holiday - left the chickens shut in their run with my parents to check on them every other day. Just under a week later and the youngest chook was dead. Then when we got home, we found the other two dead. Apart from the first chicken who was always a bit sickly looking, the others were all in excellent healthy, laying well etc - what could have killed them all within a week? I'm particularly worried because we've another four (two hens and two cocks) that we hatched a few months ago - they're in a different enclosure to the others so I want to make sure whatever it was that killed the others doesn't pass to this lot. I don't know if it's probative but we have runner ducks who are in the area too.

Any ideas?
Thanks