How did my gerbil die?

Question
Jen,
I currently own eleven gerbils. I have had two litters and they are now separated by gender into three large cages. I wouldn't consider myself a reckless breeder as I did not allow for the creation of any animals than I could personally keep and thoroughly care for. I love each of them very much and would never do anything to harm them. That being said, I hope you will offer me your knowledge to help me understand a recent tragedy within my gerbil family. A few days ago, I returned from a two week school trip and found that one of my males from the first litter born on 6/6/07 had died sometime while i was gone. I don't know how long he had been dead, but he was in his house, and my mom never noticed it. He lived with his father and two brothers, one of which has appeared sick for about a month (clicking breathing, ruffled fur, very small and thin) but seems to be improving since I started adding ornacycline to their water. Before I left, I ran out of ornacycline, so while I was gone they had regular water. They are on aspen bedding, so I don't know why they would have respiratory problems. Anyway, Beau (the one who died) was perfectly healthy before I left and was very big and active compared to his sickly brother who is still alive and well. When I found his body, the sides of his belly were  hairless, puffed out and dark purpleish. His belly also looked a little raw, but I didn't want to look too closely. He's only about eight months and should be too young for a scent gland tumor, and the affected area doesn't seem to be focused around his scent gland. I've cleaned the cages since I found him but I'm worried his brothers and father could have been affected by being around the body for however long. Also, if it's a disease, should I be worried about his family? Is it possible a family member killed him? They've gotten into little fights but never done any serious damage. I've also considered that the others in the cage could have caused the concerning appearance after he died (kinda gross to think about). Any insight into why he might have died or any precautions I should take to ensure the safety of his family is very much appreciated. Thanks!

Answer
Dear Hallie, Sorry about the late response, my computer was messed up so I couldn't even log into all experts until now.
I'm very sorry about the loss of your gerbil. (Good luck with one that  you said seemed sick before)
It seems to me the most logical answer to this tragedy would be fighting.
Since we are not sure of the cause of death, we can't be sure whether or not the other gerbils were affected. Keep a close eye on them and if you notice anything take them to a vet.
 I apologize again for the late reply. :/