Every significant disease can have longstanding impacts. Damage is often cumulative. Actual causation can come via many routes. One person might have diminished physical fitness due to weeks/months laid up with the disease. Another might have impact from various drugs/treatments administered during the disease. Others might come away with diminished organ functionality (heart/kidneys etc) that in turn stresses other systems. The "what doesn't kill you" mantra is just that. It isn't based in science. It is almost always better to avoid getting sick in the first place.