This is very interesting (and rather frightening!) as I lost all but 2 of my herd over night a couple of years ago.
The temperature was stable, they were all in the same pens (pairs mostly) inside an old stable where they had been for months if not years. I noticed a couple of the oldies were a bit slow to their greens, and that a couple had drank more water than they usually do, but it was summer and I didnt think any more about it. The next day - around mid-day - my partner went into the stable to give everyone grass and found 10 of my 12 dead, just lay there, some with food in their mouths, some stretched towards thei water and some just 'asleep'. They were all in strong rigor, I called my vet who said to take the bodies in, which I did, but they were all too gone to do a PM.
Shane and Squirrel were the only 2 to survive, and at 5 were the oldest pigs other than Cookie who was also 5. All the others were 3 or under, and one little rescue girl was thought to be around 6 months.
We still have no idea what it was that killed them, but the vet put it down to a virus of some sort as Shane and Squirrel were absolutely fine, and have never shown any signs of problems.
Its rather odd how it seemed to skip those two.
I hope there are some answers soon, my heart goes out to everyone who has lost piggies xx
) , hay from a lovely farm source which is soft, dust and weed free (meant for their horses! The guinea pigs far prefer it), Burgess Excell nuggets, obviously clean water (we do put vit c solution in this) and veg. 
