MS is not a fatal disease. In general, what is meant is the following:

Secondary symptoms of MS are complications that can arise as a result of the primary symptoms. For example, bladder dysfunction can cause repeated urinary tract infections. Inactivity can result in loss of muscle tone and disuse weakness (not related to demyelination), poor postural alignment and trunk control, decreased bone density (and resulting increased risk of fracture), and shallow, inefficient breathing. Immobility can lead to pressure sores. While secondary symptoms can be treated, the optimal goal is to avoid them by treating the primary symptoms (Source: National Multiple Sclerosis Society). These secondary systems can make other, more dangerous problems likely e.g., falling.