You know, it is infectious, but it's like a lot of other diseases that tend to infect critters that are, for one reason or another, susceptible. The problem usually only spreads if you have other healthy meeces in close contact with the scabby one. There are a lot of diseases and conditions that are just waiting to get a start, and when one meece has enough concentration of whatever it is, then the mousie can become a Typhoid Mary. Mycoplasma is like that too, only a lot harder to get rid of as the fungus is always there in small quantities. Mange is more managable. there is one other skin problem that is called cold tissue disease which starts when there's too little humidity, which dried out the skin on 'cold' parts like the tail, the ears, sometimes the feet. The thing starts as an irritation from dry air, is aggravated by scratching and licking skin that cracks and get infected. Broad spectrum antibiotics and Revolution worked for me in half the case I've seen. In some others the problem resulted in amputation of part of the tail, or sorse, it spread to the base of the tail onto the torso, and in those cases euthanzsia was the only option.
Many breeders don't treat anything like this, but isolate the affected individual, and let nature takes it's course. All other things being equal, half the time the problem will rectify itself, and the other half, the poor dear has to put down. It's a judgement call, and you might want to see a vet, although, in my experience most vets don't know diddly squat about mousie illnesses. Things might be different where you are, what with the mouse fancy being wider spread in Britain.
Good luck, and let me know how things turn out, OK?