This graph illustrates the number of new cases of tuberculosis in the United States since 1985. Many researchers attribute the sharp increase in the early 1990s to the spread of acquired immunodeficiency syndrome (AIDS). People with AIDS have weakened immune systems and are particularly susceptible to contagious diseases such as tuberculosis. Poorly supervised treatment of tuberculosis also led to an increase in drug-resistant strains of the bacteria that cause tuberculosis, furthering the spread of the disease. Renewed emphasis on control and prevention has brought the incidence of tuberculosis to record low levels.