A common cause of chronic respiratory illness in mice and rats is an organism called Mycoplasma pulmonis. More to come about this at a future date. If the mouse is suffering from an asthma-type allergy, then normally only one in a cage will be affected, and it will not seem ill apart from its sneezing.
Try a different bedding, and avoid sawdust, shavings or hay. Shredded paper or CareFresh, or a similar safe bedding, is the best option. In the case of contagious diseases, often the mouse will show other symptoms besides sneezing – for example, it may be hunched up, its eyes may be dull, its fur rough. Mice can succumb very quickly to life-threatening diseases, so prompt action is necessary.
First of all, you need to work out whether the illness has been caught recently, or whether it is something that the mouse has succumbed to after carrying it for a long time. If the mouse has recently come into contact with new mice, at a show, or after new mice were introduced to the cage, then the illness could be highly contagious. Separate the affected mice from the healthy ones.
Animals can carry infections for up to a month after symptoms have disappeared, so do not let any mice which have come into contact with the ill mice mix with mice from another cage for that period. If the mouse/mice which developed symptoms had not cone into contact with any strange mice in the last few weeks, then they may well have been carrying the disease for some time.
Many organisms are opportunistic and can only cause illness when the immune system is not functioning at its normal level. Mycoplasma pulmonis can be an example of this – affected animals often live with others who carry the organism, but do not become ill.
If this is the case, you should not separate the affected mouse from its companions – putting a sick mouse into solitary confinement will often accelerate its decline.
If the companions have already been exposed to the same organisms as the sick mouse, leave them all together.
If they’re going to catch the bug, they will catch it whether you separate them or not.