My piggy had an URI a few months ago.
He went from about 3 pounds to 1.6 in just a SHORTSHORTSHORT time. He was fine one day, I was admitted to the hospital for 3-4 days, was released to go home and I picked him up and I could feel every bone in his body.
Had an emergency appointment made with the closest guinea pig vet (around here, treating 2 guinea pigs a week is the best you get, and that's what I got) the next morning, and they gave him some kind of medicine and he got all better in 2-3 weeks and very quickly gained his weight back.
He's a sassy, plump piggy again.
My pig got his from his dusty aspen bedding.