There have been a number of definitions for GWS, but perhaps the most comprehensive of them can be found at Wikipedia:
Gulf War syndrome (GWS), also known as Gulf War illness (GWI), is a chronic multisymptom disorder affecting returning military veterans and civilian workers of the Gulf War. A wide range of acute and chronic symptoms have been linked to it, including fatigue, muscle pain, cognitive problems, rashes and diarrhea.
The percentage of veterans of that conflict who ultimately suffered from GWS was astounding: More than one-third, or approximately 250,000 of the 697,000 vets who served in the first Gulf War campaign, are afflicted, according to the National Academy of Sciences.
In 2009, a study published in the Journal of Inorganic Biochemistry posited that that the substance aluminum hydroxide could be the primary GWS culprit, as documented at Science.NaturalNews.com:
Gulf War Syndrome is a multi-system disorder afflicting many veterans of Western armies in the 1990-1991 Gulf War. A number of those afflicted may show neurological deficits including various cognitive dysfunctions and motor neuron disease, the latter expression virtually indistinguishable from classical Amyotrophic lateral sclerosis (ALS) except for the age of onset. This ALS "cluster" represents the second such ALS cluster described in the literature to date. Possible causes of GWS include several of the Adjuvants in the Anthrax vaccine and others. The most likely culprit appears to be Aluminum hydroxide.
The study, conducted by Canadian researchers from the Department of Ophthalmology and Visual Sciences, University of British Columbia, Vancouver, involved a set of experiments in which researchers examined the potential toxicity of aluminum hydroxide in male mice that had been injected subcutaneously with two doses that were equivalent to that given to humans.
Later, researchers examined the animals' spinal cord and motor cortex using immunohistochemistry:
Aluminum-treated mice showed significantly increased apoptosis of motor neurons and increases in reactive astrocytes and microglial proliferation within the spinal cord and cortex. Morin stain detected the presence of Aluminum in the cytoplasm of motor neurons with some neurons also testing positive for the presence of hyper-phosphorylated tau protein, a pathological hallmark of various neurological diseases, including Alzheimer's disease and Frontotemporal Dementia.Multiple additional causes of GWS?
In a second set of experiments, scientists injected mice with six doses of aluminum hydroxide and then conducted behavioral analyses, which "revealed significant impairments" in several motor functions, as well as diminished memory capacity.
The ground combat phase of the Gulf War lasted a mere 100 hours and began in late February 1991, but troops were on the ground in a toxic environment in Kuwait and Iraq for several months. As such, from 1995 to 2005, the health of combat vets got worse compared to military members who had not been deployed to the combat zone.
Those affected experienced a host of problems ranging from new chronic diseases to functional impairment, repeated clinic visits and hospitalizations, chronic fatigue and related symptoms, posttraumatic stress and other problems.
In addition to the 2009 study, other research has suggested that the syndrome may also have been caused by depleted uranium (which was used by U.S. tanks against Iraqi tanks, as well as U.S. Air Force planes like the A-10), sarin gas, smoke emanating from oil wells that had been set on fire by retreating Iraqi forces, predeployment vaccines and psychological factors. That said, only two substances - pyridostigmine (an antitoxin for nerve agents) and organophosphate pesticides have been strongly, and routinely, associated causally with Gulf War Syndrome, according to the U.S. Department of Veterans Affairs.