mouse death

Question
QUESTION: hi
my mouse, which i had for less than 3 months, died suddenly last night. i had been away since thurs, being valentines, and my mom was looking after it, but i know it wasnt becos of poor care.

My story...
I brought 2 mice (mouse A, being the one who died, and mouse B) from my college, [i am doing an animal care course and they have an animal unit blah blah blah and t hey breed fancy mice]. i took them as soon as i could after their birth and they came from the same litter.
Mouse A was very lively and loved being handled, he was the mose active of the lot. within a few weeks mouse B was fully grown, but mouse A dint grow to no more than 3 maybe 4cm.i had also noticed that he would make funny sounds, as if he was sneexing or coughing, and his nose would be red as if he'd been sctratching alot, and would be worse some days than others. i never noticed any fighting between the two mice but recently i thought they may be, so i split them up. it was till recently that i noticed mouse A was squinting alot and not just cos hed just woken up! he also use to have a habit that if the cage lid was low enough he would jump to try and escape but would bang his head, nearly knocking himself out, and he'd do this repeatedly, i had to but a different cage as he wouldnt stop. he seemed like a normal healty mouse other than that though, he was always active and ready to be handled, im sure he even knew his own name! so his sudden death really confuses me, it cant be anything contagious as mouse B is fine! as he always has been.
my guesses are that he could have been born with an illness or maybe a tumour could have caused this, but i never found any lumps or bumps, even after his death.

so if anyone can help, it will be much appriciated, i am just completely baffed at this and cant seem ta figure out why it happened.

thanks for reading
:)

ANSWER: Dear Lisa,
thank you for your question.
I suspect that the mouse had some kind of birth defect, the small size might be a sign of that. At the very least he had a respiratory infection because of the sneezing, but that usually doesn't kill them that fast, unless it was very serious and interfered with the mouse's development. Sneezing and coughing are symptoms that need to be treated, such things can turn into a chronic illness very fast. www.rmca.org/Vets has a list of mouse vets.
If the surviving mouse if female, I recommend getting another female to keep her company, they should be kept at least in pairs.
I hope I was of some help to you
Jennifer

---------- FOLLOW-UP ----------

QUESTION: thankyou that was helpful,
i didnt think anything of the sneezing or coughing untill i read about it yest.
but the other mouse is male, they were both male, do i leave him on his own? i doubt i'd be able to get one from the college again.

Answer
The problem with male mice is that they don't get along. Brothers may sometimes live together peacefully, but even they often fight. Introducing two males that have never seen each other doesn't work. If you can find a vet who will neuter the mouse, he can live with females (after four weeks, they can still mate successfully some time after the neutering).

An alternative is keeping him together with two or more deer mice, spiny mice,  or natal rats, so that he has at least some company, even if it's not the same species. Both spiny mice and natal rats need a pretty big cage, though, and are more for watching that anthing else. Deer mice (white footed mouse, Peromyscus leucopus, P. polionotus and P. maniculatus are the most common species in captivity) are very active and interesting to watch, they are in my opinion the best option for co-housing with a male fancy mice. The species cannot interbreed. Mare sure you get captivitybred mice since wild deer mice amy carry the Hanta Virus which can be transmitted to humans and is potentially deadly.
http://www.centralpets.com/animals/mammals/rodents/rod2492.html

I hope I was of some help to you
Jennifer