Cold temperatures weaken the immune system and that allows pathogens to set in and the cause the infection; in addition to this, there are those specialized "nasties" that seem to be able to sub due even the perfect set of lymphocytes. I have never heard of soaking, but there are about a million different ways to go about solving any problem I suppose. Mild cases can usually be solved at home by elevating temps and I always elevated humidity as well, but the former is a greater priority; however, since it is spreading, your most effective route will be prescription antibiotics, Baytril is quite commonly prescribed, RI is pretty common so any vet around you should have enough knowledge to help you out or at least have a a colleague that could guide him or her. Until you can get to a vet though make sure you can stabilize those temps to about 90F and make sure you are washing your hands well.
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