my rat died suddenly:(

Question
i had three rats last week one of my rats died from old age. One of my other girls became very depressed and she wouldn't move so very much. she would eat every now and then, and she would move to drink some water. But no matter what she would go straight to the same spot and would curl up and lay there alone. We would give both our other girls lots of attention and then tonight when i was about to say goodnight to her, i found her lying still in the same spot not breathing; she was dead. What could have killed her suddenly?

Answer
Its very hard to know what could have killed her...but you said your other rat died from old age. Can you tell me more about what the other rat acted like prior to her passing?

When rats lose their cage mates, depression often sets in. When they are depressed, they are stressed of course, which in turn leads to a weakened immune system. This can cause illness very fast. Some vets may assume the rat caught something from the cage mate that had passed away but often it is not related to that and simply due to stress causing the immune system to break down. Basically we say the rat died of a broken heart.  She could have developed an infection that caused her death.  It can be confusing when they display symptoms that can be linked both to illness and also depression so its too hard to say what it could be from unless I am sure the first rat did not have any type of infection that may have been mistaken for old age ailments.