Army Sgt. Nick Harrison learned he was infected with H.I.V. six years ago, but the once fatal diagnosis has barely changed his routine at work or at home because he keeps the virus in check with a once-a-day pill. The only thing H.I.V. crippled was his military career.

The military bars anyone with the virus that causes AIDS from joining. Policies crafted in the 1980s allow troops who contract the disease while in the military to stay as long as they remain otherwise healthy, but bars them from deploying in nearly all cases.

The question of how to treat healthy troops who have H.I.V. was hardly an issue a few decades ago, when few with the infection lived long. Now that they can lead long and healthy lives, however, the restrictions meant to control a deadly epidemic have stifled careers. Many have been shut out from coveted specialties or denied promotions. And under a new policy that requires troops to be readily deployable or be discharged, nearly 1,823 otherwise healthy troops with H.I.V. may be forced to leave the military.

Now Sergeant Harrison, who served 18 years in the Army and National Guard and deployed to Afghanistan and Kuwait, is suing the Defense Department, arguing that the H.I.V. policies are outdated and discriminatory and have cost him a promotion to captain.