Tuberculosis (TB) is caused by the bacterium Mycobacterium tuberculosis, primarily affecting the lungs. A significant portion of the world’s population carries the bacteria in some form. Understanding the stages of this infection is important because the risk of transmission changes dramatically depending on its stage. This article clarifies the distinction between latent infection and active TB disease, focusing specifically on contagiousness.
Defining the Two Types of TB Infection
TB infection is categorized into two states: Latent TB Infection (LTBI) and Active TB Disease. LTBI occurs when the immune system successfully contains the inhaled Mycobacterium tuberculosis bacteria. The bacteria remain dormant, walled off within the body, often in the lungs. Individuals with latent TB do not feel sick, display no symptoms, and their chest X-rays are typically normal.
Active TB Disease is the state where the bacteria overcome immune defenses and multiply rapidly. This destroys tissue and causes symptoms like a persistent cough, fever, unexplained weight loss, and night sweats. While TB most often affects the lungs, it can also attack other body parts like the kidney, spine, or brain.
Contagiousness: Latent vs. Active TB
Latent TB Infection is not contagious. People with LTBI cannot spread the TB bacteria to others because the microbes are contained and not being expelled from the body. Transmission does not occur through shaking hands, sharing food or drink, or touching surfaces used by someone with latent TB.
Active TB Disease is transmissible, but only under specific circumstances. The bacteria spread through the air when a person has active disease in their lungs or throat (pulmonary or laryngeal TB). When an infected person coughs, sneezes, speaks, or sings, they propel tiny, airborne droplets containing the bacteria into the environment. A person nearby must inhale these droplets to become newly infected.
If the active TB disease is limited to other organs, such as the spine or kidneys, it is not contagious. Untreated infectious pulmonary TB can potentially infect 10 to 15 other people through close contact over a year.
Why Treatment for Latent TB is Still Necessary
Even though LTBI is not contagious, medical intervention is necessary due to the risk of progression to active disease. Untreated latent infection carries a 5% to 10% lifetime risk that the dormant bacteria will become active and cause illness.
The risk of progression is higher for people with weakened immune systems, such as those living with HIV. Treating the latent infection is a preventive measure designed to kill the dormant bacteria. Eliminating the bacteria removes the individual’s risk of developing active TB disease and prevents them from becoming contagious.