So sorry to here Coco is not feeling well. In general young ferrets are pretty rugged, and there aren't a lot of illnesses that they would find in a typical home. They can be affected by respiratory illnesses, so if we're sneezing or coughing or showing discharge from eyes or nose, this can be the case. They even can catch the flu! Sometimes they will have illness that doesn't have much in the way of symptoms other than lethargy and, if you were able to assess, a fever. If there are no wounds or discharges, the most likely thing is a virus, and these typically are something we just have to let run their course. Eating and drinking are obviously hugely to our advantage, and while I'd hate to promise anything, sometimes we just take it day by day.
Antibiotics would really only be of value if we knew there was a bacterial issue, and this is much less likely unless we have eye or nose discharges or a wound.
The big issue I find with these mischevious little things is eating things they shouldn't. They aren't necessarily going to vomit, and as long as they're eating, it is still very possible that whatever might have gone down will work it's way through. If they stop eating, act painful in the belly, it is a very common thing for ferrets to need an x-ray and possible surgery to remove strange objects.
With this said, we must be really careful about any human medicines that might have been inadvertently dropped. They are sneaking and will often eat things before they think of it. Since they are small, drugs can have huge effects on them. Again, if they're eating and drinking, we have that to our advantage, and maybe the effects of some antihistamine or other dropped medicine will just wear off over the next day or so. If you think a medicine might have been ingested, it is best to contact your vet and/or poison control ASAP to see if this can be cared for.
Finally, sometimes they just feel low, like we all do. It can be difficult to figure this out, but in a young animal, major health issues such as adrenal disease or uterine cancers or pancreatic tumors become less likely. If we keep seeing bouts, having blood sugar checked would be worth doing. Insulinomas are a ferret thing, and while Coco is pretty young for this, you can't count on ferrets to follow the text book.
Hope this helps. If you have any other thoughts or questions that I might be able to clarify or add information, please don't hesitate to ask.
---- Dr. Will