Disease-modifying treatment is a term that is growing more general, referring to treatments that do not simply relieve symptoms — even disabling symptoms — but arrest or reverse the underlying pathologic process. Another way of describing such treatments is that they are not purely "symptomatic".
As a term, it probably originated in rheumatology, which designated the class of disease-modifying anti-rheumatic drugs (DMARD) initially used for rheumatoid arthritis. DMARD, especially before the introduction of non-steroidal anti-inflammatory drugs (NSAID), were rarely started early in a disease process, as they tended to have substantial risk, but could also slow progression of selected inflammatory diseases such as systemic lupus erythematosis and mixed connective tissue disorder. In rheumatology, DMARD were not appropriate for diseases that are basically mechanical degeneration, such as osteoarthritis.
Agents in this class often do not have any immediate effect on the patient's symptoms, and, indeed, the first external manifestations of their use may be side effects. Their beneficial effects may only become apparent over time, when the symptomatic disease slows or stops its progression. Safety monitoring, usually with appropriate laboratory tests but sometimes specialized examinations (e.g., periodic opthalmological examinations when antimalarial drugs are used as rheumatologic DMARDs), is often appropriate.