WASHINGTON — Amid ongoing fears of an Ebola outbreak in the United States, Americans got a grim reminder on Tuesday about the ongoing public health threat posed by another deadly virus: HIV. Seventy percent of Americans who have HIV do not have the disease in check, and many of them are no longer receiving treatment, according to a study published Tuesday. HIV, or human immunodeficiency virus, causes AIDS, a syndrome that destroys the body’s ability to fight off infections.