Tuberculosis, a disease fueled by poverty, overcrowding and undernutrition, is a global barometer of deprivation. Of the more than 4,000 men, women and children TB kills every day, most are poor.

TB’s connection to poverty is explained, in large part, by the way Mycobacterium tuberculosis operates: It causes disease in only a fraction of the people it infects. Almost a quarter of the world’s population harbors the tuberculosis bacterium, but in most cases, it is walled off by the immune system.

The bug activates in 5-15 percent of carriers, who tend to have other health problems. They may be undernourished, infected by HIV, have a habit of smoking or drinking too much, or live with extreme air pollution. They may suffer from diabetes or other chronic illness, or take immunosuppressant medications. A loss of sun-supported vitamin D can be a trigger in people who migrate from equatorial areas to places where there is winter. In many cases, scientists can’t tell exactly what has roused the pathogen.

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