Epidemiology is defined as the study of how often diseases occur in different groups of people and why. It also investigates all the factors that determine the presence or absence of diseases and disorders. About one-third of the global population (nearly 2 billion) is infected with tuberculosis (TB) and 95% of deaths due to TB are in the developing world. It is the second leading cause of death from an infectious disease worldwide after human immunodeficiency virus (HIV). Tuberculosis has been classically associated with poverty, overcrowding, and malnutrition. Mostly the young adults in their productive years are affected. Low income countries and deprived areas (particularly slums), within big cities in developed countries, present the highest TB incidences and TB mortality rates. The six countries having the largest number of cases of 2013 in 2013 were India, China, Nigeria, Pakistan, Indonesia, and South Africa. An estimated 1.5 million TB deaths occurred in 2013, of which 1.1 million was among HIV-negative people and 360,000 among HIV-positive people. About 480,000 people developed multidrug-resistant tuberculosis (MDR-TB) in the world in 2013. More than half of these cases were in India, China, and the Russian Federation.