For what it's worth, I have a cat who was really sick as an 8 week old kitten. We had rescued her when she was about 4 weeks and she had tons underlying issues (worms, diarrhea, eye infection) so definitely taken from her mom too soon and weak immune system, but we did our best to nurse her back to health. In Nov 2018 she was diagnosed with Corona (one of the strains, I don't know which one, 1st time I ever heard of it) and he said there wasn't much we could do since it's a virus, but he did prescribe amoxicillan and we found she got better within 4 doses, she started to turn around from near death. When asked why antibiotics would help treat a virus, he said antibiotics have a natural way of boosting the immune system to fight off a virus while also preventing secondary infections. I don't see why this isn't normal practice for treating viruses like these.