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  • Tuberculosis, or TB, is one of the oldest and most common infectious diseases. About

  • one third of the world population is believed to be infected with TB. Fortunately, only

  • about 5% of these infections progress to active disease. The other 95% of infected people

  • are said to have a dormant or latent infection; they do not develop any symptoms, and do not

  • transmit the disease. Tuberculosis is caused by a rod-shaped bacterium,

  • or a bacillus, called Mycobacterium tuberculosis. An infection is initiated following inhalation

  • of mycobacteria present in aerosol droplets discharged into the atmosphere by a person

  • with an active infection. The transmission process is very efficient as these droplets

  • can persist in the atmosphere for several hours and the infectious dose is very low

  • less than 10 bacilli are needed to start the infection.

  • Once in the lung, the bacteria meet with the body’s first-line defense - the alveolar

  • macrophages. The bacteria are ingested by the macrophages but manage to survive inside.

  • Internalization of the bacilli triggers an inflammatory response that brings other defensive

  • cells to the area. Together, these cells form a mass of tissue, called a granuloma, characteristic

  • of the disease. In its early stage, the granuloma has a core

  • of infected macrophages enclosed by other cells of the immune system. As cellular immunity

  • develops, macrophages loaded with bacteria are killed, resulting in the formation of

  • the caseous center of the granuloma. The bacteria become dormant but may remain alive for decades.

  • This enclosed infection is referred to as latent tuberculosis and may persist throughout

  • a person's life without causing any symptoms. The strength of the body’s immune response

  • determines whether an infection is arrested here or progresses to the next stage. In healthy

  • people, the infection may be stopped permanently at this point. The granulomas subsequently

  • heal, leaving small calcified lesions. On the other hand, if the immune system is compromised

  • by immunosuppressive drugs, HIV infections, malnutrition, aging, or other factors, the

  • bacteria can be re-activated, replicate, escape from the granuloma and spread to other parts

  • of the lungs causing active pulmonary tuberculosis. This reactivation may occur months or even

  • years after the initial infection. In some cases, the bacteria may also spread

  • to other organs of the body via the lymphatic system or the bloodstream. This widespread

  • form of TB disease, called disseminated TB or miliary TB, occurs most commonly in the

  • very young, the very old and those with HIV infections.

  • Tuberculosis is generally treatable with antibiotics. Several antibiotics are usually prescribed

  • for many months due to the slow growth rate of the bacteria. It’s very important that

  • the patients complete the course of the treatment to prevent development of drug-resistant bacteria

  • and re-occurrence of the disease.

Tuberculosis, or TB, is one of the oldest and most common infectious diseases. About


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B2 中上級

結核(TB)。疾患の進行、潜伏感染と活動性感染。 (Tuberculosis (TB): Progression of the Disease, Latent and Active Infections.)

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    Study English に公開 2021 年 01 月 14 日