IN 1917, a psychiatrist and neurologist called Constantin von Economo arrived at a meeting of the Vienna Psychiatric Society to describe a strange malady that had been spreading sporadically through Europe.

The disease - a form of encephalitis, or brain inflammation - had first appeared in Romania in 1915, and was characterised by fever, headache, intense lethargy, sleep disturbance and abnormal eye movements.

Von Economo described seven severe cases, including two which had been fatal. Around 40 cases had also been reported among soldiers in northern France.

In the subsequent decade incidence exploded.

Between 1916 and 2027, the epidemic resulted in at least 65,000 known diagnoses, but is estimated to have sickened millions worldwide.

READ MORE: Private long Covid clinic 'utterly overwhelmed' as patients told there is 'nothing on NHS'

The condition, dubbed encephalitis lethargica (EL) or "sleepy sickness", appeared to have a mortality rate of around 30 per cent in the acute stage but around half of survivors were left with brain damage that eventually degenerated into post-encephalitic Parkinson's Disease.

In some cases sufferers were plunged into a coma-like state, motionless and unable to speak, while others exhibited behavioural changes such as psychosis.

As suddenly as it appeared, however, the disease began to peter out.

The last known case was reported in 1940, though academic interest in its long-term effects continued.

The Herald: Constantin von EconomoConstantin von Economo (Image: Austrian National Library)

In 1969 an unexpected breakthrough occurred when Oliver Sacks, a gifted neurologist based in New York, discovered that a new psychoactive drug, L-DOPA - which the body converts into dopamine - had the power to "awaken" these patients from their sleep-like state.

After decades of being moribund and institutionalised in hospitals across Britain and North America hundreds of patients - who Sacks compared to "extinct volcanoes" - suddenly erupted into life as if nothing had happened.

Sacks, who died in 2015, later said it was the "most significant and extraordinary moment" in his life.

READ MORE: 'Some days I can't even lift a cup to my mouth' - life for the Scots with long Covid 

Yet more than a century on from that original outbreak of encephalitis lethargica (EL), the question of what exactly caused it remains unanswered.

The leading hypothesis, still unproven, is that it was a post-viral syndrome triggered by infections with a particularly virulent form of Spanish flu which swept the globe after the First World War.

But correlation is not causation and others considered EL to be a standalone disorder.

Tests of archived brain samples in 2001 also failed to find evidence of the influenza virus.

Writing in the journal Epidemiology and Infection in 2009, Philip Mortimer, an expert in viruses at the former Health Protection Agency in London, said it was "doubtful whether the cause of EL will ever definitely be established, at least until a possible recurrence of it in epidemic form".

The Herald: Oliver SacksOliver Sacks (Image: Getty)

The lingering mystery was highlighted again this week as discussions turned to long Covid and a critical report from Holyrood's Covid-19 Recovery Committee, which criticised - among other things - the "stigma" faced by patients and the "apparent lack of any lessons being learned" from chronic fatigue syndrome (also known as ME).

In a phone-in on BBC Radio Scotland, Dr Chris Smith, a Cambridge University virologist known for his 'Naked Scientist' podcasts, noted that medical science has known for decades that a minority of people suffer catastrophic and lasting consequences in the wake of viral infections.

He cited the surge in encephalitis lethargica following the 1918 Spanish flu pandemic as an example where "in the aftermath of a flu respiratory infection, in some susceptible individuals, you can get are knock-on consequences for other organs including the brain".

While the exact cause of EL is still disputed, the underlying point is not: post-viral syndromes are real, and have probably existed for millennia.

What set Covid apart was the sheer number of infections occurring in an extremely condensed time period - impossible before mass air travel - and in an era when social media could raise awareness and science is better-placed to investigate it.

The Herald: It remains unclear why some people are susceptible to developing long Covid symptoms, and why some recover while others do notIt remains unclear why some people are susceptible to developing long Covid symptoms, and why some recover while others do not (Image: PA)

As Dr Smith put, this meant it "frothed over and reached public consciousness and medical and scientific consciousness and got looked at" as opposed to being a phenomenon "unique to coronavirus infection".

The end result may be that research stimulated as a result of the pandemic can finally unlock the mechanism behind these conditions, which have been shamefully neglected until now.

In the meantime, however, long Covid sufferers - like ME patients before them - are still stuck in a limbo with neither diagnostic tests nor cures.

Some patients eventually recover, while others do not, but it is unclear why.

READ MORE: Two thirds of Scots with long Covid face at-work discrimination

Mystery remains as to what long Covid actually is: an autoimmune condition, reactivation of latent viruses, or a form vasculitis where blood vessels have become inflamed?

There is still no consensus definition for long Covid symptoms, and disagreement as to whom it covers.

For example, does a patient with lasting organ damage as a result of an intensive care battle with Covid count - or does it only apply to patients who developed debilitating symptoms such as fatigue and pain following a mild infection?

The lack of any diagnostic test combined with the wide range of ailments associated with it, and the fact that statistics tend to be based on self-reported cases, mean the condition has been easy to dismiss as psychosomatic.

That it disproportionately affects women only fuels that scepticism.

The Herald: Long Covid continued to affect nearly 2 million people in the UK as of December last yearLong Covid continued to affect nearly 2 million people in the UK as of December last year (Image: ONS)

According to a Lancet report in March, however, 10-20 per cent of people will experience some form of long Covid (unexplained symptoms four or more weeks on from a known Covid infection) and one in 10 eventually stop working.

In Scotland, more than 90,000 people say they have had long Covid symptoms for longer than a year.

While research suggests that both the Omicron strain and vaccines have reduced the risk to an individual of developing long Covid compared to the early stages of the pandemic, at a population level this benefit may be outweighed by the sheer prevalence of the virus.

Like Sacks' "extinct volcanoes", however, long Covid patients are all too easy to ignore: often housebound, bedbound, and out of sight. 

A tragic reminder of a pandemic the rest of society is eager to forget.