Hospitalism

Hospitalism, or anaclitic depression, is caused by temporarily or permanent loneliness. ‘In medical and psychologic science the term “Kaspar-Hauser Syndrome” is used for the most severe form of hospitalism because of the complete lack of stimuli in combination with wrong treatment or rather cruelty. ¹⁾’
Most rodents are social. That’s why isolation from conspecifics and therefore the lack of social stimuli necessarily lead to hospitalism. Symptoms are sometimes difficult to notice and also very different from one individual to another. Some individuals simply die sooner than they would have when having friends, some refuse to eat properly, some are much tamer than they should be. Often they move less than conspecifics living in a group. Some rodents develop contact behaviour disorders which makes it more difficult to introduce them to other mice later on. Unnatural aggressions or fear might also be a sign of hospitalism. The most familiar occurrence of hospitalism may be the stereotype behaviour of animals in a zoo who move in the same scheme again and again for hours. Something similar you can notice at rodents at home like constant gnawing at the cage bars.
Because some of these symptoms may have another cause and also because of the broad variety of symptoms hospitalism is difficult to make out though it is an obligatory result of isolation.
Some symptoms you won’t recognise until you eliminate the cause and look back on the behaviour of the individual before getting friends.
Because it is so difficult to recognise the symptoms, many owners of lonely mice think their pets are happy. But they aren’t! Even though the owner plays with his/her pet every day that is not substitute for conspecifics because humans have a totally different language to mice and the lonely mouse has no one to talk to. Apart from that, it is always alone most of the day.
Beside the loneliness the lack of stimuli is another factor to cause hospitalism. If an enclosure lacks of sufficient stimuli, changes will be recognised also at groups. These changes will disappear only if you offer a bigger and/or better structured enclosure.
Even though often not obvious for everybody, hospitalism is one of the worst problems which exists in the field of behavioural disorders because the victims suffer immensely psychically.

Sources
Wikipedia (DE)

Translation
Angelus Noctis
Proofreading
Jedediah