The Institute of Medicine reviewed the evidence for treatments for symptoms associated with Gulf War syndrome and related conditions. They concluded that selective serotonin reuptake inhibitors, serotonin–norepinephrine reuptake inhibitors and cognitive behavioural therapy were most likely to be beneficial to patients.

A catastrophic illness is a severe illness requiring prolonged hospitalization or recovery. Examples would include coma, cancer, leukemia, heart attack or stroke. These illnesses usually involve high costs for hospitals, doctors and medicines and may incapacitate the person from working, creating a financial hardship. They are the type intended to be covered by high-deductible health plans. Research indicates that the unusual economic environment of the delivery of catastrophic illness care encourages the use of innovative therapies. Medicare contains a benefit for catastrophic illness.

Multisymptom illness is more prevalent in Gulf War veterans than veterans of previous conflicts, but the pattern of comorbidities is similar for actively deployed and nondeployed military personnel.

Notable definitions include:

- Centers for Disease Control and Prevention (CDC) definition (1994), the most widely used clinical and research description of CFS, is also called the Fukuda definition and is a revision of the "Holmes" or "CDC 1988" scoring system. The 1994 criteria require the presence of four or more symptoms beyond fatigue, while the 1988 criteria require six to eight.

- The ME/CFS 2003 Canadian Clinical working definition states: "A patient with ME/CFS will meet the criteria for fatigue, post-exertional malaise and/or fatigue, sleep dysfunction, and pain; have two or more neurological/cognitive manifestations and one or more symptoms from two of the categories of autonomic, neuroendocrine, and immune manifestations; and the illness persists for at least 6 months".

Clinical practice guidelines are generally based on case descriptions, with the aim of improving diagnosis, management and treatment. An example is the CFS/ME guideline for the National Health Services in England and Wales, produced in 2007 by the National Institute for Health and Clinical Excellence (NICE).

There are no characteristic laboratory abnormalities to diagnose CFS; testing is used to rule out other conditions which could be responsible for the symptoms. When symptoms are attributable to certain other conditions, the diagnosis of CFS is excluded. As such, a diagnosis of CFS/ME is generally one of exclusion (of alternative diagnoses).

Timothy F. Jones, of the Tennessee Department of Health, recommends the following action be taken in the case of an outbreak:

- Attempt to separate persons with illness associated with the outbreak.

- Promptly perform physical examination and basic laboratory testing sufficient to exclude serious acute illness.

- Monitor and provide oxygen as necessary for hyperventilation.

- Minimize unnecessary exposure to medical procedures, emergency personnel, media or other potential anxiety-stimulating situations.

- Notify public health authorities of apparent outbreak.

- Openly communicate with physicians caring for other patients.

- Promptly communicate results of laboratory and environmental testing to patients.

- While maintaining confidentiality, explain that other people are experiencing similar symptoms and improving without complications.

- Remind patients that rumors and reports of "suspected causes" are not equivalent to confirmed results.

- Acknowledge that symptoms experienced by the patient are real.

- Explain potential contribution of anxiety to the patient's symptoms.

- Reassure patient that long-term sequelae from current illness are not expected.

- As appropriate, reassure patient that thorough clinical, epidemiologic and environmental investigations have identified no toxic cause for the outbreak or reason for further concern.

Some responses by authorities to MPI are not appropriate. Intense media coverage seems to exacerbate outbreaks. Once it is determined that the illness is psychogenic, it should not be given credence by authorities. For example, in the Singapore factory case study, calling in a medicine man to perform an exorcism seemed to perpetuate the outbreak.

Jones compiles the following symptoms based on their commonality in outbreaks occurring in 1980–1990:

Chronic critical illness is a disease state which affects intensive care patients who have survived an initial insult but remain dependent on intensive care for a protracted period, neither dying nor recovering. The most characteristic clinical feature is a prolonged requirement for mechanical ventilation. Other features include profound weakness associated with critical illness polyneuropathy, increased susceptibility to infection, metabolic changes and hormonal changes. There may be protracted or permanent delirium, or other marked cognitive impairment. The physical and psychological symptoms of the disease are very severe, including a propensity to develop post traumatic stress syndrome.

Strict definitions of chronic critical illness vary. One definition is the requirement for mechanical ventilation for 21 days or more. It is estimated that 5-10% of patients who require mechanical ventilation as part of their initial illness will go on to develop chronic critical illness. Overall prevalence has been estimated at 34.4 per 100 000 of the population. Most adult patients do not survive chronic critical illness, and furthermore even those who are discharged from hospital frequently die soon after discharge. One-year mortality in adults is 48-68%. However, children fare better with two-thirds surviving to 5 years or beyond.

Frail elderly people are at significant risk of post-surgical complications and the need for extended care. Frailty more than doubles the risk of morbidity and mortality from surgery and cardiovascular conditions. Assessment of older patients before elective surgeries can accurately predict the patients' recovery trajectories.

The most widely used frailty scale consists of five items:

- unintentional weight loss >4.5 kg in the past year

- <20th population centile for grip strength

- self-reported exhaustion

- low physical activity such that persons would only rarely undertake a short walk

- slowed walking speed, defined as lowest population quartile on 4 minute walking test.

A healthy person scores 0; a very frail person scores 5. Compared to non-frail elderly people, people with intermediate frailty scores (2 or 3) are twice as likely to have post-surgical complications, spend 50% more time in the hospital, and are three times as likely to be discharged to a skilled nursing facility instead of to their own homes. Frail elderly patients (score of 4 or 5) have even worse outcomes, with the risk of being discharged to a nursing home rising to twenty times the rate for non-frail elderly people.

Psychogenic disease (or psychogenic illness) is a name given to physical illnesses that are believed to arise from emotional or mental stressors, or from psychological or psychiatric disorders. It is most commonly applied to illnesses where a physical abnormality or other biomarker has not yet been identified. In the absence of such "biological" evidence of an underlying disease process, it is often assumed that the illness must have a psychological cause, even if the patient shows no indications of being under stress or of having a psychological or psychiatric disorder.

Examples of diseases that are believed by many to be psychogenic include psychogenic seizures, psychogenic polydipsia, psychogenic tremor and psychogenic pain.

There are problems with the assumption that all medically unexplained illness must have a psychological cause. It always remains possible that genetic, biochemical, electrophysiological or other abnormalities may be present which we do not have the technology or background to identify.

The term psychogenic disease is often used in a similar way to psychosomatic disease. However, the term "psychogenic" usually implies that psychological factors played a key causal role in the development of the illness. The term "psychosomatic" is often used in a broader way to describe illnesses with a known medical cause where psychological factors may nonetheless play a role (e.g., asthma can be exacerbated by anxiety).

Among the indigenous peoples of Latin America, in which this illness is most common, susto may be conceptualized as a case of spirit attack. Symptoms of susto are thought to include nervousness, anorexia, insomnia, listlessness, fever, depression, and diarrhea.

Susto may be a culturally dependent variation of the symptoms of panic attack, distinct from anxiety and depressive disorders.

Survivor guilt (or survivor's guilt; also called survivor syndrome or survivor's syndrome) is a mental condition that occurs when a person believes they have done something wrong by surviving a traumatic event when others did not. It may be found among survivors of murder, terrorism, combat, natural disasters, epidemics, among the friends and family of those who have died by suicide, and in non-mortal situation. The experience and manifestation of survivor's guilt will depend on an individual's psychological profile. When the "Diagnostic and Statistical Manual of Mental Disorders IV" (DSM-IV) was published, survivor guilt was removed as a recognized specific diagnosis, and redefined as a significant symptom of post traumatic stress disorder (PTSD).

Mild disease can be treated with fluids by mouth. In more significant disease spraying with mist and using a fan is useful. For those with severe disease putting them in lukewarm water is recommended if possible with transport to a hospital.

In the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision (DSM-IV-TR), paranoid, schizoid, and schizotypal personality disorders may be diagnosed as conditions "premorbid" to the onset of schizophrenia.

In psychology, premorbidity is most often used in relation to changes in personality, intelligence or cognitive function.

Changes in personality are common in cases of traumatic brain injury involving the frontal lobes, the most famous example of this is the case of Phineas Gage who survived having a tamping iron shot through his head in a railway construction accident.

Declines from premorbid levels of intelligence and other cognitive functions are observed in stroke, traumatic brain injury, and dementia as well as in mental illnesses such as depression and schizophrenia.

Other usage in psychology include premorbid adjustment which has important implications for the prognosis of mental illness such as schizophrenia. Efforts are also being made to identify premorbid personality profiles for certain illness, such as schizophrenia to determine at risk populations.

Prevention includes avoiding medications that can increase the risk of heat illness (e.g. antihypertensives, diuretics, and anticholinergics), gradual adjustment to heat, and sufficient fluids and electrolytes.

Chronic illness can affect a child’s development at any stage. During infancy and childhood chronic illness can be detrimental to the development of secure attachment, interpersonal trust, self-regulation, and/or peer relation skills. During middle adolescence, chronic illness can prevent a child from being in school on a regular basis. This can affect a child’s academic and social competence. During adolescence, chronic illness can affect the development of autonomy and self-image. It can also interfere with peer & romantic relationships, and the desire for independence can lead to poor treatment compliance.

Questionnaires and checklists such as the Beck Depression Inventory or the Children's Depression Inventory can be used by a mental health provider to help detect, and assess the severity of depression. The Seasonal Pattern Assessment Questionnaire can be used to screen for seasonal affective disorder. Semi structured interviews such as the Kiddie Schedule for Affective Disorders and Schizophrenia (KSADS) and the Structured Clinical Interview for DSM-IV (SCID) are used for diagnostic confirmation of depression.

Definitive diagnosis relies on a blood test for alcohol, usually performed as part of a toxicology screen.

Law enforcement officers in the United States of America often use breathalyzer units and field sobriety tests as more convenient and rapid alternatives to blood tests.

There are also various models of breathalyzer units that are available for consumer use. Because these may have varying reliability and may produce different results than the tests used for law-enforcement purposes, the results from such devices should be conservatively interpreted.

Many informal intoxication tests exist, which, in general, are unreliable and not recommended as deterrents to excessive intoxication or as indicators of the safety of activities such as motor vehicle driving, heavy equipment operation, machine tool use, etc.

For determining whether someone is intoxicated by alcohol by some means other than a blood-alcohol test, it is necessary to rule out other conditions such as hypoglycemia, stroke, usage of other intoxicants, mental health issues, and so on. It is best if his/her behavior has been observed while the subject is sober to establish a baseline. Several well-known criteria can be used to establish a probable diagnosis. For a physician in the acute-treatment setting, acute alcohol intoxication can mimic other acute neurological disorders, or is frequently combined with other recreational drugs that complicate diagnosis and treatment.

Diplomatic illness is the practice amongst diplomats and government ministers of feigning illness, or another debilitating condition, to avoid engaging in diplomatic or social engagements. The excuse of ill-health is designed to avoid formally offending the host or other parties. The term also refers to the period during which the "diplomatic illness" is claimed to persist.

Some common childhood onset chronic illnesses are cystic fibrosis, cerebral palsy, juvenile diabetes, juvenile rheumatoid arthritis, and cancer. Chronic illness is often a risk factor for developing psychopathologies, due to the psychological toll it takes on the children and their developing brains. Approximately 10 million children in the United States suffer from a childhood onset chronic illness.

Depressed mood may not require professional treatment, and may be a normal temporary reaction to life events, a symptom of some medical condition, or a side effect of some drugs or medical treatments. A prolonged depressed mood, especially in combination with other symptoms, may lead to a diagnosis of a psychiatric or medical condition which may benefit from treatment. Different sub-divisions of depression have different treatment approaches.

In the United States, it has been estimated that two thirds of people with depression do not actively seek treatment. The World Health Organisation (WHO) has predicted that by 2030, depression will account for the highest level of disability accorded any physical or mental disorder in the world (WHO, 2008).

The UK National Institute for Health and Care Excellence (NICE) 2009 guidelines indicate that antidepressants should not be routinely used for the initial treatment of mild depression, because the risk-benefit ratio is poor. A recent meta-analysis also indicated that most antidepressants, besides fluoxetine, do not seem to offer a clear advantage for children and adolescents in the acute treatment of major depressive disorder.

Initial screening for CIP/CIM may be performed using an objective scoring system for muscle strength. The Medical Research Council (MRC) score is one such tool, and sometimes used to help identify CIP/CIM patients in research studies. The MRC score involves assessing strength in 3 muscle groups in the right and left sides of both the upper and lower extremities. Each muscle tested is given a score of 0-5, giving a total possible score of 60. An MRC score less than 48 is suggestive of CIP/CIM. However, the tool requires that patients be awake and cooperative, which is often not the case. Also, the screening tool is non-specific, because it does not identify the cause a person's muscle weakness.

Once weakness is detected, the evaluation of muscle strength should be repeated several times. If the weakness persists, then a muscle biopsy, a nerve conduction study (electrophysiological studies), or both should be performed.

If a person with ILI also has either a history of exposure or an occupational or environmental risk of exposure to "Bacillus anthracis" (anthrax), then a differential diagnosis requires distinguishing between ILI and anthrax. Other rare causes of ILI include leukemia and metal fume fever.