The incidence of tuberculosis of the lungs, the "white plague" that formerly affected millions of people, declined from the 1950s until 1984; sanatoriums were closed and routine screening was abandoned in the United States. Then, between 1984 and 1992, the incidence increased by 20%, chiefly because of immigration from countries where it is common and because of AIDS, which leaves people particularly vulnerable to the disease. Renewed efforts at control and advances in treatment have been rewarded with incidence declines each year, amounting to a total decline of 31% from 1992 to 1998.
Worldwide the outlook has been far less encouraging. In 1993 the World Health Organization (WHO) declared TB a global health emergency. Approximately one third of the world's population is infected; an estimated 1.3 million died in 2012. The vast majority of new cases occur in sub-Saharan Africa. Spread of TB is especially rapid in areas with poor public health services and crowded living conditions. In homeless shelters and prisons, crowded conditions and inadequate treatment often go together. Areas where living conditions are disrupted by wars, famine, and natural disasters also are heavily affected.
Especially alarming has been the spread of drug-resistant strains of TB. By the late 1990s scientific experts and international health officials warned that drug-resistant strains were spreading faster than had been anticipated. Bacteria can survive and become drug resistant in patients whose treatment is not properly monitored and seen to completion. Multidrug resistant (MDR) TB strains are resistant to two or more of the commonly prescribed first-line drugs, while extensively drug resistant (XDR) strains are also resistant to three or classes of the more toxic second-line drugs. Some believe that unless major new treatment strategies are initiated in source countries, drug-resistant TB will eventually become epidemic even in areas with good control programs, such as Europe and America. In 2011, WHO estimated that there were more than 80,000 cases, many of them undiagnosed, of drug-resistant TB in Europe.