For virtually all types of infections, the number of persons who develop clinical disease, and who subsequently seek diagnosis and treatment, is far smaller than the number of persons who have asymptomatic, or subclinical, infections. For example, a symptomatic infection with the Human Immunodeficiency Virus (HIV), is about ten times more common than diagnosed cases of Acquired Immune Deficiency Syndrome (AIDS). However, the AIDS virus can be transmitted by infected persons even if they are not clinically ill. Therefore, employing isolation precautions only for patients with diagnosed cases of AIDS would miss the vast majority of persons who harbor this infectious agent.