from Part I - Systems
Published online by Cambridge University Press: 15 December 2009
INTRODUCTION – AGENT
Mycobacterium tuberculosis is a large, nonmotile, curved rod that causes the vast majority of human tuberculosis cases. M. tuberculosis and three very closely related mycobacterial species (M. bovis, M. africanum, and M. microti) all cause tuberculous disease, and they comprise what is known as the M. tuberculosis complex. M. tuberculosis is an obligate aerobe, accounting for its predilection to cause disease in the well-aerated upper lobes of the lung. However, M. tuberculosis can persist in a dormant state for many years even with a limited oxygen supply. The organisms also persist in the environment and are resistant to disinfecting agents.
Mycobacterium species are classified as acid-fast organisms because of their ability to retain certain dyes when heated and treated with acidified compounds. Humans are the only known reservoir of infection.
EPIDEMIOLOGY
Tuberculosis is the second leading cause of death related to an infectious disease. Nearly one-third of the world's population is infected with Mycobacterium tuberculosis. In 2005, the World Health Organization (WHO) estimated there were 8.8 million new cases of tuberculosis and 1.6 million deaths due to the disease. Tuberculosis is the leading cause of death among human immunodeficiency virus (HIV)-infected persons, accounting for 12% of worldwide deaths. Whereas the average person infected with Mycobacterium tuberculosis has a 10% lifetime chance of developing active disease, immunocompromised patients can have their risk jump to that same percentage annually.
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