Proper quarantine must be observed for any mouse to be shown or sold/traded at a show in the US. The length of time varies from club to club (from 3 weeks to 4 weeks to 5 weeks) but they must be in separate airspace, i.e. separate houses, not just different rooms in the same house. Many serious fancers have bought free-standing sheds or converted a detached garage to serve this purpose. This "no shared airspace" requirement is because deadly diseases like Sendai, Murine Hepatits, SDA, and others may travel in the vents of homes for short periods and reach mice who are a few rooms away. Quarantine also entails that no new rodents come into the airspace for that same amount of time. There is no real way to enforce this so we have to take members at their word. Of course if you don't have a separate house, another room is better than nothing at all and we tell people this.
While in QT all my mice get Baytril (a generalist antibiotic) and Iver-On (a generalist parasiticide), as a precautionary measure, if I think they need it. If they come from one of the 2 breeders I trade with often, they often get nothing or are not quarantined at all because I have known them a while and trust them fully. If they come from a show where there were lots of animals, they're quarantined no matter where they originally came from.
Once you get to the show or event, there is also an official health checker who does a mini-physical on every mouse present before it is allowed to enter. If that means 400 mice have to be looked at, so be it. I'm a health checker with one of the clubs I belong to. And mice are sometimes (though not often) turned away. The thing is, when one mouse is turned away, that person's entire stock cannot be shown or brought into the room.
Some say we're really paranoid about our quarantine and it's probably true but I've seen disastrous things happen and whole varieties vanish from the US in part at least because of failed quarantine.
Since I don't have a separate house in my apartment, I quarantine in my car. This can only be done when it's not too hot, of course.
Like Sarah, when I do have a sick mouse I almost always euthanize it immediately because it's just not worth losing my stock that I've worked so hard on by keeping a sick mouse around and letting it spread the disease to all the others.