Prevalence is highest in conditions of poverty and overcrowding.
The interaction between the TB epidemic and the HIV epidemic is lethal. Unlike HIV, the TB germ can spread through the air. So individuals with active tuberculosis are contagious to those with whom they come into close contact. If left untreated for a year, one individual can typically infect 10 to 15 other people. In the USA, a longstanding annual decline in TB cases ended abruptly in 1985, at the peak of HIV spread.