Mice are also very social animals, living in colonies in the wild, so they do not do well as a solitary pet. Keeping a same-sex pair (litter mates usually do well together) is necessary.
Mice can also contract a serious bacterial infection called Tyzzer's disease. Symptoms include diarrhoea, loss of appetite and listlessness. Mice can get diet related health problems so make sure to feed them a low fat diet, avoid sunflower seeds as these are very unhealthy for a mice.