Introduction

Tuberculosis (TB) is the name given to a spectrum of clinical syndromes caused by a small number of mycobacterial species of the Mycobacterium tuberculosis complex. Most of the other ~130 described mycobacterial species are environmental organisms of which a few can cause disease only in very specific circumstances. These are principally associated with impaired host defense. M. tuberculosis, and less commonly the other organisms of the M. tuberculosis complex, M. bovis, M. africanum and M. microti, by contrast, can affect immunocompetent individuals, and are highly effective pathogens. Evidence to support this statement comes from history, including archeological evidence of TB in bony human remains from 500 000 years ago. That one-third of the current world's population is estimated to be infected with TB is further evidence. In addition, the global human immunodeficiency virus (HIV) epidemic amplifies the importance of tuberculous infection, as co-infection is quite common.

Infection with M. tuberculosis may lead immediately to progressive clinical illness, but it may also be asymptomatic or cause a minimal self-limiting illness. The organism then lies dormant in macrophages (latent infection). In 10% of such latent cases the disease reactivates. Clinical tuberculosis can affect any organ system, but the lungs and lymph nodes are most commonly affected in the UK.