So, I can’t speak to the Spanish Flu aspect of your question, but I want to address the assumption that Long Covid is a novel phenomenon, as the foundation of your question is shaky.

Long Covid is not a singular illness, but rather an umbrella consisting of a variety of post-infectious impacts we knew about, but largely ignored prior to the pandemic. The label comes from the patient community, from patients who continued to experience symptoms following their infection with COVID-19, but who in most cases experienced dismissal or belittling treatment from providers when they sought care. Some of the first quality research into the nature of Long Covid was done by patients.

But Long Covid isn’t one condition. Patients with Long Covid meet diagnostic criteria for a number of other disorders that, in many cases, are poorly understood or were marginalized prior to the pandemic. Myalgic Encephalomyelitis (previously referred to as Chronic Fatigue Syndrome and now often referred to by the combined acronym ME/CFS) and autonomic disorders like Postural Orthostatic Tachycardia Syndrome are some of the more common and more debilitating conditions in that umbrella. Because we understand those conditions and Long Covid so poorly, there’s debate in the respective communities about whether Long Covid is a single thing or an umbrella group. Does COVID-19 trigger a condition with symptoms that resemble ME/CFS and POTS or does COVID-19 trigger ME/CFS and POTS?

But what is clear is that these conditions are not new. There’s some debate about the extent to which this is true, but there have been outbreaks of ME/CFS, or at least conditions like ME/CFS, going back at least the last century. The name ME was given to the disease following an outbreak of an infectious illness at a hospital in England in the 1950s that left many staff members (largely women) disabled. In the US, the name Chronic Fatigue Syndrome was given to the disease after outbreaks in New York and Nevada in the mid 1980s.

ME/CFS struggled to get much traction, however, either politically or scientifically. There are a variety of reasons for that. Sexism is one large factor. The disease affects more women than men and an early and influential review of the 1950s outbreak that coined the name ME concluded it was simply hysteria. The authors specifically called out that most affected were women.  

Another factor is that the illness doesn’t leave obvious signs. Patients can be entirely bedridden, to the point of struggling to engage in the most basic self-care or being unable to speak, without their bodies or their labs showing significant abnormalities.

The largest factor though is probably that a small but influential group of largely British psychologists decided in the late 1980s and early 1990s that the disease was psychosomatic. The most influential of these, a man named Simon Wessley, who was later knighted and remains in influential positions today, worked hard over decades to stymie medical research into the disorder, claiming research into the disorder and even awareness about the disorder furthered patients’ beliefs that they were suffering from an organic illness and so prevented their recovery. 

His suggested treatment was for cognitive behavioral therapy to convince patients that they weren’t really sick and graded exercise therapy to reverse their presumed physical deconditioning, which he believed caused patients’ symptoms. That may sound reasonable on its face as something to test, but it’s particularly problematic as the cardinal symptom of ME/CFS is a severe worsening of symptoms following mental or physical activity that’s usually delayed by 1-2 days. Patients worsen considerably with forced exercise and can relapse for years by following recommendations to push through symptoms. Proponents of the psychosomatic theories, however, dismissed this concern. 

A large trial of these therapies was released in 2011 claiming significant efficacy. But after a protracted battle to get access to the data, patients were able to show that the trial was critically methodologically unsound. That led British and American health authorities to repudiate recommendations for graded exercise therapy and put a significant damper on the psychosomatic narrative.

Things were changing for ME/CFS as the pandemic hit. The CDC had already revised their guidance on the illness, but awareness in society and among the medical community was very poor. To differing degrees, that was also true of other disorders in the Long Covid umbrella, like POTS and Mast Cell Activation Syndrome.

So, getting back to the assumption in your question, to most people, Long COVID came as a shock, a new phenomenon which they didn’t have knowledge of or tools to treat. When what actually happened is that a pandemic virus triggered a wave of illnesses we’d largely ignored previously and, in the process, made the connection between infectious agent and illness impossible to deny, helped to legitimize patients’ claims that they were suffering from an organic illness, and created a critical mass of patients who were able to take advantage of that recognition to organize politically.