. I like it over here in the ferret forum, but it's normally pretty inactive. So, your questions:
The realistic lifespan of a ferret is anywhere from about 4-8 years. It's longer when you buy from a good breeder. I've heard of a 10 year old, but a 15 year old is next to impossible -- though there may have been an incredibly lucky fuzzbutt.
For ferret diseases -- they all seem fated to get something rather troublesome/costly to maintain later on in life. The most common and/or feared diseases of ferrets are lymphoma, adrenal disease, and aleutian's disease. They can also have many digestive problems since their digestive system is so small and has a horrible time digesting anythng fibrous. There are many more, but these are the basics you'll want to read up on(google them).
For food info, refer to these threads, they cover quite a bit:
Carissa's posts -- lots of info)
(read my posts near the end -- guidelines on how to pick a food if you're just not sure)
Marshalls -- this is a huge topic...but I'll do my best. Marshall's is a ferret mill that produces ferrets both for in-home pets and to sell to research laboratories as guinea pigs of science. They mass produce ferrets, but in a way they also contribute to a ton of medical and veterinary research by doing what they do. Of course, I wish it weren't the ferrets they had to sacrifice. A lot of their practices are arguably unethical. That is, selling both for research and pets -- also not ensuring that Petco(their main seller) keeps ferrets in the correct condition after being sent off. They have shown time and time again that they really just want the money. The majority of ferrets in the US come from Marshall's ferret farm/ferret mill -- and a lot of people say that the overpopulation of ferret shelters is due to Marshall's and other large ferret mills(Path Valley Ferrets is another one).
The ferrets are always descented and desexed. Some say it's bad to desex and descent so early on in life. They ship ferrets out at the age of 6-8 weeks...sometimes you can still find them in the pet stores with shaved bellies from their surgeries. And, some also believe that descenting altogether is an unnecessary and cruel procedure.
Anyways, my overall opinion about them is that they definitely don't have the wellbeing of ferrets in mind. It's weird, however, because I don't really approve of private breeders either. My future ferrets will be adoptees. I understand that animals from private breeders tend to live long, healthy lives -- but I just really love ferrets. I'd like to adopt one since it was probably abandoned, even if it ended up having numerous health problems. I'd take care to the best of my ability till the day it died. No matter what. Btw, I'm not saying that someone who chooses to buy a Marshall's ferret or ferrets from a private breeder doesn't love ferrets. Sometimes it's the only choice, and of course, everyone has different ideas of love for animals.
My three ferrets are from Petco...aka Marshall's ferrets. They're all about a year old, and I haven't had any real health problems just yet(thankfully). Others have had Marshall's fuzzies that died within 2 years of age or even less. The people who I've heard that from blame Marshall's for the death of the ferret. I'm really not so sure...but it's definitely possible.