What does a positive tuberculin skin test indicate?

A positive tuberculin skin test, also known as the Mantoux test, indicates that a person has been exposed to the mycobacteria that causes tuberculosis (TB). This test measures the immune reaction to a purified protein derivative (PPD) of the TB bacteria. When a person has been exposed to TB, their immune system develops a memory of the bacteria, and upon re-exposure to the PPD, a noticeable skin reaction occurs at the injection site.

It is important to understand that a positive result does not confirm that a person has active tuberculosis disease. Instead, it signifies that the immune system has encountered the bacteria at some point, which could be a result of a past infection or exposure in a latent form where the bacteria are present but not causing active disease. Further testing, like a chest X-ray or sputum tests, would be required to determine if TB is active.

The other answer choices do not accurately describe the implications of a positive test. For instance, having active tuberculosis would require additional clinical investigation and tests beyond just the skin test, any immunity to TB is not indicated by just this test alone, and a viral infection is unrelated to the test's purpose or its results.