Tuberculosis epidemiology is the field of science that is concerned with the study of health and disease within populations and the various factors that lead to these conditions, with a goal of preventing the spread of disease. One-third of the world's population has been infected with M. tuberculosis. Anyhow, not all infections with M. tuberculosis lead to TB disease. 13.7 million Chronic active cases, were reported by 2007 and 8.8 million new cases were estimated in 2010 and 1.45 million deaths, are often noted in developing countries. Out of these, 0.35 million deaths occur in those co-infected with HIV. Tuberculosis distribution is not symmetrical across the globe, in many African and Asian countries (about 80% of the population) test positive in tuberculin tests, and in U.S only 5–10% of the population test positive. In 2012, around 450,000 people developed MDR-TB. TB incidence varies with age. In Africa, TB chiefly affects adolescents and young adults. However, in United States, TB has gone from high to low incidence, where TB is mainly a disease of older people, or of the immunosuppressed.