Transmission occurs through the airborne spread of infectious droplets. When an infectious person coughs, sneezes or spits, they propel TB bacteria into the air. Left untreated, a person with active TB can infect an average of 10 –15 people each year. Exposure to M. tuberculosis can lead to infection that is asymptomatic and non-contagious – referred to as latent TB. One in ten people not infected with HIV but infected with TB will become sick with active TB in their lifetime. Half of those who become infected with TB and progress to the development of active TB will do so within two years of infection, while in the others it occurs much later or not at all. People living with HIV are at a much greater risk of developing active TB once infected, which increases as the degree of immune suppression increases.