Historical Context:
The term "Myalgic Encephalomyelitis" was first used in the 1950s to describe an outbreak of an illness with symptoms resembling poliomyelitis, where patients experienced muscle pain (myalgia) and signs of central nervous system involvement (encephalomyelitis), suggesting inflammation of the brain and spinal cord.

Symptom Emphasis:
ME is often emphasized to involve neurological symptoms, including neuroinflammation, indicating a more specific involvement of the nervous system. This definition may stress the presence of neurological signs and symptoms, such as cognitive impairments, autonomic dysfunction, and post-exertional neuroimmune exhaustion, which is a profound worsening of symptoms following even minor physical or mental exertion.

Systemic Exertion Intolerance Disease (SEID) is a term proposed by the Institute of Medicine (IOM) in its 2015 report as a new name for Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS).

The recommendation to adopt SEID aimed to more accurately describe the condition and its primary symptom: a profound intolerance to exertion. The proposal intended to address the stigma and misunderstanding associated with the name "Chronic Fatigue Syndrome" and to highlight the systemic nature of the illness and the significant impact it has on patients' ability to engage in daily activities.

Key Criteria for SEID:

The IOM report specified diagnostic criteria focusing on the core symptoms of SEID, which include:

1. Significant reduction or impairment in the ability to engage in pre-illness levels of activity (work, education, social, or personal activities) that persists for more than six months, accompanied by fatigue that is often profound, is not the result of ongoing excessive exertion, and is not substantially alleviated by rest.

2. Post-exertional malaise (PEM):
A worsening of symptoms following physical, cognitive, or emotional exertion that would have been tolerated before the illness onset.

3. Unrefreshing sleep.

Additionally, to meet the diagnostic criteria, a patient must experience either cognitive impairment or orthostatic intolerance. These criteria aimed to make diagnosis more straightforward and to ensure that the illness is taken seriously by healthcare providers, patients, and the general public.

Purpose and Reception:

Purpose: The proposal to change the name to SEID and adopt new diagnostic criteria was part of an effort to improve clinical care, increase awareness, and stimulate research funding for ME/CFS by providing a more accurate and descriptive name.

Reception: The reception to the term SEID has been mixed within the ME/CFS community and among medical professionals. Some have welcomed it as a step forward in acknowledging the disease's severity and systemic nature. Others have preferred to stick with the established terms of ME or CFS, citing concerns about the potential for SEID to obscure the historical and clinical understanding of the illness.

Historical Context:
 The term "Chronic Fatigue Syndrome" emerged in the United States in the 1980s when clusters of patients were experiencing chronic, unexplained fatigue not attributable to other known medical conditions.

Symptom Emphasis:
CFS definitions have traditionally focused on chronic fatigue as the core symptom, accompanied by a broad spectrum of other symptoms, including but not limited to sleep disturbances, muscle and joint pain, headaches, sore throat, and impaired memory or concentration. The diagnostic criteria for CFS often include a list of symptoms that must be present for a diagnosis and the exclusion of other medical and psychiatric conditions that could explain the fatigue.

In recent years, the terms have been combined into ME/CFS by many health organizations and researchers to acknowledge the overlap in symptoms and the historical contention around naming. The combined term aims to respect both the fatigue and the neurological symptoms experienced by patients.

The use of ME/CFS seeks to encapsulate the complexity of the illness, recognizing it as a systemic disease with a significant impact on the nervous system, immune system, energy metabolism, and more.

The diagnostic criteria for ME and CFS have varied over time and across different health authorities. The most commonly referenced criteria include the 1994 Fukuda Criteria (for CFS), the 2003 Canadian Consensus Criteria (for ME/CFS), and the more recent 2015 Institute of Medicine (now the National Academy of Medicine) Criteria (for ME/CFS), each with slightly different emphasis on the range and severity of symptoms.

In summary, while ME and CFS may have originated from different historical contexts with somewhat different emphases on symptomatology, their current use, especially in the combined form ME/CFS, acknowledges the considerable overlap in symptoms and the need for a comprehensive understanding of the illness to advance research, diagnosis, and treatment.