Sorry to hear about Gus ... the first thing would have been to sterilize everything in the cage ... anything that could not be sterilized should be disposed of a.s.a.p./yesterday.
Do you, your neighbors, family, friends have a cat? Is there a stray cat in the neighborhood? Feline distemper is cat pneumonia ... it is highly contagious ... if you walk across an area where a cat has been or pet a cat with feline distemper or one who is a carrier of feline distemper (carries the virus, but never gets sick), the virus/germ/bacteria can easily be passed on to a chinchilla. Chinchillas usually die within 24 to 48 hours when they have pneumonia.
I have a stethoscope I use to listen to my little ones ... it is one used for infants or small children. I have been lucky had have not had it in my herd, but know of others who have lost a/or more chins to pneumonia.
Do you have a water bottle and not a bowl of water for the chin to drink from? If a chin gets wet and chilled, he can become sick quickly.
Is your cage made of wire? No more than 2 solid sides (opposite to each other) is recommended for chinchillas. The air needs to be able to flow normally through a cage, but not blow through a cage from a vent, a fan or a window. Enclosed cages (such as an aquarium holds all the fumes from the urine and feces close to the bottom of the 'cage' and the chin has no choice but to breath that bad air and become sick from it.
I would have had Oliver checked out ... just for safety's sake and for peace of mind.
Jo Ann