The mortality rate is the percentage of patients who reach a stage of a disease who will die of that disease or the complications of that disease, even with the most appropriate treatment. For example, the mortality rate for a patient with the common cold is 0%, while the mortality rate for a patient who shows any of the symptoms of rabies is 100%.
In some diseases, such as some types of cancer, the mortality rate is calculated over a time period - typically five years. This is typical for any disease that responds to aggressive treatment. For example the mortality rate for AIDS is still 100%. However, the five year mortality rate keeps improving on a regular basis, as the disease can be contained with treatment.
In many cases, effective treatment and correct diagnosis are often worked into the mortality rate, and there are some diseases, such as Brucellosis, which may or may not respond to best treatment, but may resolve themselves even without treatment. For example, whether or not a person survives Guillain-Barre syndrome is only partly dependent on proper treatment. In many cases, the disease resolves itself without treatment, or even well treated patients diagnosed early will die of the disease.