Better is a vaccine that is bread on chicken embryocelcutures. If he does not have these he can order them. And further The flue, also a virus and a Ferret can become very ill from it, fortunately he will usually recover from it, in a quit environment with enough attention. A common cold is also something a ferret can get. There are a number of diseases a ferret can obtain, for example because it ate raw porkmeat it could get Aujeszky disease.
Bacteria can be dangerous for a ferret also.
I also run in to an other thing. Toxoplasmoses, whish is a cats disease, Pregnant women are advised not to clean the litterbin during their pregnancy, the change of contamination is not ruled out. Our Ferret can not become ill from this bacteria but might be a carrier. This means that you should not clean the ferretbin while your are pregnant. To bad for al those men who now wil have to clean two litterbins.