GWS: Theories of Origin
There are, broadly, two competing theories: the more common view--and the one mostly endorsed by the government--is that GWS is a form of post-traumatic stress disorder. Elaine Showalter, Tina Brown Professor of Profitable Writing at Princeton, even likened it to Victorian hysteria in her Hystories.
The other view is that GWS is a result of exposure to chemical, biological, and/or radiological agents. The majority of people suffering from the illness believe this to be the case. A recent study bolsters this claim by demonstrating a correlation between incidents of Lou Gehrig's disease and Gulf War service.
This is not meant to be a comprehensive overview, nor am I suggesting that the two theories above are necessarily mutually exclusive.
Anthrax Mailer: Theories of Motivation
As Dr. Rosenberg notes, the most puzzling clue available is why the mailer chose Daschle and Leahy as targets. Some have speculated, contra Rosenberg's thesis, that the mailer wanted to precipitate a massive national crackdown on civil liberties as a pretext for a militia-led revolution. Daschle and Leahy, Senate Majority Leader and Judiciary Chairman respectively, would thus seem logical targets. Leahy, who has been slightly critical of Ashcroft's policies since 9/11, might seem a crucial convertee of any right-wing immiseration strategy.
One of the problems with this theory is the lunacy it implies seems incommensurable with the expertise needed to acquire the anthrax. Recent reports that the Capitol Hill anthrax is identical to that manufactured by the Army in recent years. The U.S. military, controversially, began its anthrax vaccination program in 1998. Rosenberg notes that the mailer had to have a vaccine to handle the material safely. These factors point towards a military source.
The letters themselves, while handled with apparent forensic care, do not seem too sophisticated. FBI investigators determined very quickly that it was likely the writer of the letters was attempting to portray himself (almost certainly a man, according to FBI behavioral profile) as Middle-Eastern.
Assume that the mailer was an Army microbiologist or lab technician with access to anthrax. He likely had the material stockpiled before 9/11. The first letters were postmarked on 9/18. Were the letters themselves written during this time? The Capitol letters vary slightly from the media letters (and are more ill-tempered) and have not (to my knowledge) yielded any further forensic clues. The target of the misdirection would have to be Iraq, mainly, as it is known to the mailer (who addressed one of the letters to NY Times bioterror expert Judith Miller) that they would be the prime suspects for foreign origin.
Why Iraq? While a number of possible explanations could be offered, the most fitting is that the mailer either has GWS or is deeply disturbed by those who do. He blames Iraq for exposing U.S. troops to whatever it was, and the U.S. govt. for attempting to cover it up. The anthrax mailings would force the government to come to terms with biological terrorism and the potential epiphenomenon of Iraq's destruction would also be welcome.
If, as the FBI believes, a lone person was responsible for the mailings, the only hope of locating him would be the necessarily restricted access to materials the crime involved. A Kaczynski-style family tip is also possible. Considering GWS as a motivational factor may yield important clues.