My mouse has died and her sister is left will she be ok?

Question
Hi Tamarah,
I brought two mice about a year ago from my local petstore, mickey and minnie, minnie being the mouse that died ): I know that mice dont live too long but what is the average life-span of a mouse? Before Minnie died she was breathing heavily and sat still alot of the time. I noticed this in Mickey too so i put a fan near the cage in case they were getting too hot. Anyway this morning i woke up to find Minnie lay in the corner of her cage breathing very heavily more-so than before so again i put the fan near her. I went out for a jog and on my return found her to be no longer with us. Im not sure weather this was just old age that is making them become ill or weather i need to treat Mickey with any medicines? also with the loss of Minnie will i need to buy another companion for Mickey or perhaps another two so when Mickey does die of old age the others will still have each other? Please answer quickly. This is very upsetting for me, i love them very much )':
Thankyou (:

Answer
Hi Danni,

I'm so sorry for your loss.  Fans do not really help mice cool off, so in the future it would be better to seek other methods of cooling them off - only if you're sure they're overheated, though.  An overheated mouse will start to lose the fur on its face (only a little, it will start to look thinner), will appear lethargic but not hunched, and may push its head against the sides of the cage.  Since sick mice lose their ability to stay warm, however, you should strive to keep the cage between around 70 and 80 degrees Fahrenheit, never letting it fall below 65 F.

The average mouse lives anywhere from one to three years, and it depends largely on their genetics.  Pet stores are sort of rolling the dice, unfortunately - I've had mice that only made it a couple of months and one that lived a whole three years, both from a pet shop.  This worries me that something else may have been going on, but it is impossible for me to tell from here.  Is there any way you could bring Mickey in to a small animal vet?  This would be your very best course of action for Mickey, since a professional taking a real look at your mouse could pin down a problem very quickly.  If not, the only thing I can recommend is a general antibiotic called tetracycline.  It is not effective against everything, but if you absolutely cannot see a vet it would be a good first step.  If you go with the tetracycline, it must be given for at least 14-30 days.  Here are the instructions:  http://en.allexperts.com/q/Mice-3824/2010/2/Tetracycline-directions-sneezing-mou...

I am assuming Mickey is a girl, and if so then yes, she could use new cagemates.  However, since you're not sure if she's sick too and you never know what new mice may bring in, your best option would be to buy a temporary (but escape-proof!) cage and isolate your new mice until Mickey is looking better.  This should really be done with any new mice, just to minimize your chances of bringing home something no good to Mickey.  Three weeks is usually enough time to make sure no symptoms show up in your new girls, and then it would just be up to how Mickey is feeling.  Two mice would be a wonderful idea for exactly the reason you stated - if Mickey goes, there will still be two girls to keep each other company.  :)

Best of luck!
Tam