It is something of a mystery why an ME/CFS outbreak has not occurred since the early 1990s, when prior to that outbreaks occurred somewhere in the world almost yearly (see
this list of ME/CFS outbreaks). Assuming this is not just an issue involving failure to report an outbreak of symptoms (which seems unlikely), it suggests some factor in the environment may have changed since the early 1990s, which inhibits the occurrence of ME/CFS epidemics.
They have though. There was an outbreak in Norway in the 2000s that had several papers published on it. It was caused by a sewage leak into the water supply.
I think the reason why there aren’t several every few years though is simply down to sanitation. Vastly improved sanitation, both in institutional settings as well as municipal water supplies.
This is all conjecture because I wasn’t alive in the peak outbreak decades, but I have to imagine that laws, guidelines, and technology for sanitation are light years ahead of where they were in the peak outbreak years of the 1950s. Obviously the importance of sanitation was understood back then I’m sure, but the situation has changed in several ways since then. The broad overuse of antibiotics has lead to the emergence of MRSA and other potentially deadly hospital derived microbial infections. We also went through deadly pandemics of influenza in more recent history that demanded a response due to the relatively large number of even young, healthy people killed. There is also a broader awareness of these and also things like meningococcal meningitis in settings like school dorms, communal housing, and barracks, the other popular settings for historical outbreaks. The response to all of these things is a much, much larger awareness of the importance of sanitation and a huge change in human behavior to address it. The company I worked at during the 2009 flu pandemic installed hand sanitizer stations at every doorway. It’s common now to see these all over the place in public, medical or commercial settings.
If you’ve spent any time in a hospital recently, they are meticulous with hygiene. The RNs and CNAs will wear and change gloves for each individual interaction with a patient, and will hand wash or sterilize their hands between glove changes. The rooms are sterilized thoroughly on a schedule and after each patient leaves. At my local hospital, any staff member that declined a flu shot is required to wear a face mask their entire shift for the duration of flu season. The doctors barely even put their hands on patients any more, opting instead to rely on blood and imaging test results to diagnose, rather than relying on physical exam. Things like bedpans are now plastic and thrown away after each use, rather than metal that would require sterilization. All of this greatly cuts down on the vectors for transmitting infection.
Because enteroviruses spread via the fecal oral route, they absolutely thrive on and require poor sanitation conditions to spread. They are happy as a clam in water and can remain viable on surfaces for weeks if not cleaned off. Clean water and clean fomites means less enterovirus spread, bottom line.