[1] The most common diseases that require salvage therapy are HIV and various cancers.
Antiretroviral drugs (ARVs) are given to slow down the HIV reproduction, which in turn increases quality of life and survival.
As more and more mutations conferring drug resistance develop in the HIV's genome, it becomes difficult to select an ARV that will meaningfully suppress HIV replication and keep the patient's viral load low.
Salvage therapy, in this context, is the attempt to contain the replicating HIV once the usual line of treatments have been exhausted.
[4] When at least one regimen containing protease inhibitors has failed in a patient, the subsequent attempts to treat the HIV infection may be referred to as salvage therapy.