When hundreds of thousands of American troops deployed to the Persian Gulf in 1990 and 1991 in the First Gulf War, they were exposed to a variety of chemicals. These chemicals — especially when coupled with war-related stress — seem to still be affecting nearly 200,000 Gulf War veterans — or 25 to 32 percent of those who served — more than 25 years later, and the constellation of resulting symptoms has been termed Gulf War illness.