Roughly one-quarter of the world's population has been infected with M. tuberculosis, and new infections occur at a rate of one per second. However, not all infections with M. tuberculosis cause tuberculosis disease and many infections are asymptomatic. In 2007 there were an estimated 13.7 million chronic active cases, and in 2017 there were 10 millionnew cases, and 1.6 million deaths, mostly in developing countries. 300 000 deaths occurred in those co-infected with HIV. Tuberculosis is the most common cause of death from "a single infectious agent ". The absolute number of tuberculosis cases has been decreasing since 2005 and new cases since 2002. Russia has achieved particularly dramatic progress with decline in its TB mortality rate—from 61.9 per 100,000 in 1965 to 2.7 per 100,000 in 1993; however, mortality rate increased to 24 per 100,000 in 2005 and then recoiled to 11 per 100,000 by 2015. The distribution of tuberculosis is not uniform across the globe; about 80% of the population in many African, Caribbean, south Asian, and eastern European countries test positive in tuberculin tests, while only 5–10% of the U.S. population test positive. In 2017, the country with the highest estimated incidence rate as a % of the population was Lesotho, with 665 cases per 100,000 people. As of 2017, India had the largest total incidence, with an estimated 2 740 000 cases. According to the World Health Organization, in 2000-2015, India's estimated mortality rate dropped from 55 to 36 per 100 000 population per year with estimated 480 thousand people died of TB in 2015. In developed countries, tuberculosis is less common and is mainly an urban disease. In Europe, deaths from TB fell from 500 out of 100,000 in 1850 to 50 out of 100,000 by 1950. Improvements in public health were reducing tuberculosis even before the arrival of antibiotics, although the disease remained a significant threat to public health, such that when the Medical Research Council was formed in Britain in 1913 its initial focus was tuberculosis research. In 2017, in the United Kingdom, the national average was 9 per 100,000 and the highest incidence rates in Western Europe were 20 per 100,000 in Portugal. These rates compared with 63 per 100,000 in China and 44 per 100,000 in Brazil. In the United States, the overall tuberculosis case rate was 3 per 100,000 persons in 2017. In Canada, tuberculosis is still endemic in some rural areas. The incidence of TB varies with age. In Africa, TB primarily affects adolescents and young adults. However, in countries where TB has gone from high to low incidence, such as the United States, TB is mainly a disease of older people, or of the immuno-compromised. Tuberculosis incidence is seasonal, with peaks occurring every spring/summer. The reasons for this are unclear, but may be related to vitamin D deficiency during the winter.
