A recent study has identified a transcriptional signaturefor active tuberculosis, suggesting that the distinctionbetween active and latent forms may not be absolute. Our understanding of tuberculosis (TB) is heavily coloredby the available diagnostic tools. Active TB is identifiablefirst because of the presence of clinical symptoms, suchas chronic cough, fever and weight loss. Active TB maybe confirmed by the identification of the causativebacteria, Mycobacterium tuberculosis (Mtb), but in manycases bacteria are undetectable. Latent TB is defined bythe presence of a T-cell-mediated immune response toMtb in an asymptomatic individual. This immunologicalresponse to Mtb is widely believed to indicate thepresence of viable organisms that might cause activedisease in the future, although how many people with adetectable immune response actually harbor Mtb isunknown. The tests used to identify active and latent Mtbinfection are among the oldest in modern medicine,dating back to the late 19th century. Thus, much of ourunderstanding of human TB has been frozen for morethan a century.
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