This story was sad and disturbing - in a very curious way. I've had hamsters before, but never mice. The question Calcifer asked really made me very curious -
Does anyone know if a mouse that possibly ate/attacked/killed a sick mouse with myco will get infected also?
So- I looked it up.
This comes from The Merck Veterinary Manual, Eighth Edition 1998
Mycoplasma are microorganisms that produce fungus-like filamentous cell growths and have no cell wall. There are different strains of mycoplasma. In humans, the strain Mycoplasma pneumoniae causes what we call "walking pneumonia". Other strains of mycoplasma are found in pigs, birds, cattle, mice, rats, and other animals. Two types of mycoplasma found in rats and mice are Mycoplasma pulmonis and Mycoplasma arthritidis.
Most domestic rats and mice are born with Mycoplasma pulmonis in the mucous membranes of their respiratory tracts as well as in the females' genital tracts. The commonly referred to "Myco" is murine respiratory mycoplasmosis, a disease also referred to as chronic respiratory disease (CRD), or chronic murine pneumonia in both rats and mice. Mycoplasmosis is caused by M. pulmonis.
Rats and mice with M. pulmonis infections are contagious to other rats and mice, but not to humans. The infection is spread between the mother rodent and her young, by sneezes over short distances, sexual transmission, other animals which carry the disease (but are not affected by it), and in the case of genital M. pulmonis, the mother passes the infection to the young during birth.
Clinical signs, (obvious symptoms), of mycoplasmosis in the upper respiratory system include sniffling, occasional squinting, rough hair coat, reddish nasal discharge (porphyrin), and sneezing. If the inner ear becomes involved, torticollis, (head tilt), may occur.
The bronchopulmonary syndrome, (involving the lungs and heart), is either brought on by or made worse by ammonia, bacterial infections, or Sendai virus infection. It is characterized by lethargy, rough hair coat, hunched posture, chattering, weight loss, labored breathing, and eventually death. Unless bacteria complicate the
respiratory infections, the terminal clinical stages of mycoplasmosis may last weeks or months.
While clinical signs of mycoplasmosis may not be evident, M. pulmonis never leaves the animal's body. At some point in the infected animal's life, M. pulmonis will likely cause illness. The presence of M. pulmonis increases the risks of secondary infections from bacteria such as Cilia-Associated Respiratory (CAR) Bacillus and viruses such as Sendai Virus and Sialodacryoadenitis (SDA).
I suppose the answer to the question is yes. If a mouse is infected it can be transmitted to another. I agree, I wouldn't put another mouse in with the remaining one.
How sad 
I'm so sorry this happened to you. I'd be wondering what happened to the one that died too Maybe it was some kind of violent fight?
In any regard - I'm very sorry for your loss.
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