Decreased cellular immunity is the primary culprit. While the number of new cases of tuberculosis for the United States is again on the decline, largely as a result of strenuous public health control measures, the numbers continue to rise in several states and the District of Columbia, where the disease remains at near epidemic levels. The problem is compounded by shifts in world population. Forty percent of new TB cases in the United States last year occurred in foreign-born patients, up from 16 percent in 1982, according to the New York Times. Numbers are similar in most developed countries.
World travel means world spread, including resistant organisms.
A single casual exposure has a low risk. Yet researchers estimate that each person with active TB will go on to infect between nine and 12 others. Reasonable precautions should be taken. The single most preventable risk factor for TB is HIV disease. But the problem is more than personal hygiene.