A tragic...emotional day

Our tabby cat, Gabby, has been under the weather for over a week now. It all started when we adopted Chanterelle from the Humane Society. She gave all our other cats an upper respiratory infection. It seemed to be hitting Gabby the worst. Novalee has just been walking around sneezing. Poor Gabby started drooling and hadn't moved or eaten/drunken for several days. I took her to the vet on Halloween and got medication for what was then diagnosed as the upper respiratory...Gabby just being hit harder than the others. Usually this kind of thing can be kicked by a Lysine treatment which is something everybody went on a week ago and then lots of rest. Today, Saturday, Gabby's condition hadn't improved. We hadn't seen her eat or drink in days...she would sit in one spot for hour starring at the floor as she foamed at the mouth. If you moved your hand in front of her...her eyes wouldn't follow. We decided to take her back to the vet for further examining. They did blood work and x-rays to determine that she has a blood parasite that not only can be deadly, but has helped in shutting down her kidneys. I'm not even going to pretend to understand all the Vet talk...but basically some blood level thing should be between 10-30...serious issues happen at 100...and Gabby's were off the chart at 230+. Obviously this had nothing to do with an upper respiratory infection like everyone assumed. Our vet recommended sending her to the emergency center to be monitored over the weekend and put on an IV drip with medicine and nutrition. We were told that if the parasite came from a flea then she would bounce back in a few days...if the parasite was from a tick...she had maybe a day or two to live. STRESS!!!
At the emergency center...the vet there was very alarmed at her condition and seemed to disagree that this was a blood parasite. He tested her immediately for Leukemia and Aids. He said that with the blood test her kidneys had already failed and it doesn't look good.
Talk about being in shock...we knew our Gabby was sick...but we expected to take her in today mainly for some IV nutrition. Right now she is being kept at the center to see if re hydration is even possible. Kidney failure isn't really fixable...she didn't test positive to Leukemia or Aids, but we were given a grave outlook. I have to call back around 8 this evening to see if there has been any change. If not, Gabby has to be put to sleep. There is no doubt about the amount of pain she is in the Dr. warned. It's all just happening too fast to take in. I feel so helpless and ignorant to what is going on. I tried to do some research online, but have come up with no explanation. Nobody can explain why her kidneys have failed and why it all happened so quickly. At least Leukemia could have shed some light on that.