Is a relapse a different type of bodily reaction to post-exertional malaise, or just a more severe/prolonged PEM experience? I'm unsure where the term 'post-exertional malaise' originated. The CCC describes PEM as usually lasting 24 hours or longer. The word 'relapse' doesn't appear anywhere in the CCC clinical case definition. The ICC uses the term Post-Exertional Neuroimmune Exhaustion (or PENE) instead of PEM. It describes PENE as usually lasting 24 hours or longer, but defines PENE broadly so as to include relapses. Consider 3 examples: Yesterday I had to sit down for an hour or so after a relatively small exertion; Last week I was knocked around for a couple of days after a larger exertion; and In 2010 I had a relapse that lasted well over 6 months. Are these supposed to be separate physiological reactions in some way; or are the 3 simply points along a single spectrum of post-exertional reactions?