A lot of ferrets can look a bit rough this time of the year as their coat changes to winter coat. My mate has a little one that has hardly any fur at the moment did the same last year. I give mine a dash of Salmond oil every day while they are changing coat imo this helps. As other lads have said keep them dry, warm and well fed and watered and they will be fine in a few weeks.
Cheers Arry