What happened to the Female Krogan?
Eve's death is related to whether you destroyed the data in Mass Effect 2. If the data was destroyed (or not discovered at all, via not having performed Mordin's Loyalty mission in ME2 at all), then Mordin has insufficient information to keep Eve alive - she succumbs to her weakened immune system.
If Mordin has the data from ME2, then he is able to use this data to save Eve and keep her alive.
Incidentally, you can only successfully keep Mordin alive if Eve dies. If she lives, she will prevent you from successfully convincing Mordin to sabotage the cure.