Events at which large numbers of ferrets are congregated, such as ferret shows, are most likely an excellent venue for dissemination of this disease. There are a lot of unproven claims about ECE circulating on the Internet. This tends to cloud the diagnostic and treatment picture, and ultimately are damaging to ferrets and their owners. Influenza is a type of paramyxovirus, not a coronavirus. Infected animals shed the ECE virus forever.
Ferrets commonly break with the disease again after exposure to a clinically ill animal.
It is likely that immunity, if not lifelong, is certainly long-lasting. The presence of young animals with no clinical symptoms in a facility undergoing an outbreak of ECE is likely proof of this. This is just a sampling of the misinformation available on the Net. Be careful as to what you read, and make sure your information comes from a well-regarded source.