Primary Hypothesis: The primary hypothesis of this study is that antibiotic treatment directed against Mycoplasmaspecies will improve functional status of patients with Gulf War Veterans' Illnesses (GWI) who are tested as mycoplasma positive at baseline. Specifically, the primary objective of the study is to determine whether a 12 month course of doxycycline treatment in deployed Gulf War veterans presenting with symptoms of GWI and testing as mycoplasma positive improves functional status compared to placebo.