Why is my mouse's cheek swollen? Is it contagious to my other mice?

Thank you for using Petco Pet Education Center, formerly Petcoach! That particular type of mouse does not have a cheek pouch - the only mice that do have cheek pouches are deer mice (these are brown, and have a little bit more hair on the tail). This swelling can be a mass, or swelling of the lymph nodes or salivary glands which are located in that region. This can be due to a viral infection, which is very contagious. I recommend separating the mice with swellings from the mice without them as soon as possible to avoid spread of any infectious agents. The sick mice should be seen by a vet, who can prescribe antibiotic medications and supportive treatments depending on the cause. If this is caused by a virus, the affected mice may shed the virus for the rest of their lives and require life-long care and separation. I hope this helps!