A TB infection can take two forms, active and latent. Active disease is transmissible, and causes the damage to the lungs which makes TB one of the biggest killers in the world.

In the latent form, the bacteria Mycobacterium tuberculosis is quiescent and can stay that way for years until it becomes active and causes those clinical signs.

Testing for the active version of the disease is done directly, but when it comes to latency, we use the tuberculin skin test to see if someone has an immunological response - and when that happens we consider them to have latent disease.

However, in this podcast Lalita Ramakrishnan, professor of immunology and infectious diseases at the University of Cambridge; Paul Edelstein, professor of pathology and laboratory medicine at the University of Pennsylvania; and Marcel Behr, professor of medicine at McGill University question that conclusion.

Read their full analysis:
https://www.bmj.com/content/367/bmj.l5770/

Their previous analysis:
https://www.bmj.com/content/362/bmj.k2738.abstract

And search for their previous podcast - "Have we misunderstood TB's timeline?"