Tuberculosis (TB) is a serious infectious disease caused by the bacterium Mycobacterium tuberculosis. Despite common misunderstandings, TB is not hereditary; it is not passed down through a parent’s genes to a child. The disease is transmitted solely through the movement of this external pathogen from one person to another.
How Tuberculosis Spreads
Tuberculosis transmission occurs through the air when an individual with active disease in their lungs coughs, sneezes, or speaks. These actions release tiny airborne particles, known as droplet nuclei, which contain the M. tuberculosis bacteria. If another person inhales these particles, the bacteria can settle in the lungs and initiate an infection.
TB is not spread through casual contact, such as shaking hands, sharing food, or touching surfaces. Transmission generally requires prolonged, close exposure to a person who has active, untreated pulmonary TB. This explains why the bacteria are most frequently spread among household members, close friends, or coworkers who share the same air space for extended periods.
Infection Versus Inheritance
The confusion about whether TB is inherited often stems from the observation that the disease appears to run in families. When multiple family members develop tuberculosis, it is not because they have inherited a disease gene but because they have shared the same highly concentrated environmental exposure to the pathogen. They all inhabit the same enclosed space, breathing the air contaminated by the person with active TB.
An infectious disease is one caused by an external agent, such as a bacterium, that invades the body. Tuberculosis falls squarely into this category, as the infection cannot occur without the presence of the Mycobacterium tuberculosis organism. In contrast, a hereditary disease is caused by a genetic mutation or abnormality passed from parent to offspring via DNA.
The familial clustering of TB is a function of proximity and shared environment, which ensures repeated exposure to the infectious source. While an individual may inherit a genetic tendency that makes them more vulnerable to the infection, they do not inherit the disease itself. The exposure to the bacteria is the prerequisite for infection.
Factors That Increase Susceptibility
A person’s immune system is the primary factor determining whether an inhaled bacterial dose leads to an infection and, subsequently, whether that infection progresses to active disease. Not everyone who breathes in the M. tuberculosis bacteria will become infected, and only about 5% to 10% of those who are infected will develop active TB disease in their lifetime. The strength and responsiveness of the host’s immune defense largely dictates this outcome.
Numerous underlying health conditions can impair the immune system’s ability to contain the bacteria, significantly increasing the risk of developing active disease. These risk factors include conditions that compromise cellular immunity, such as HIV infection, which is the strongest known risk factor for progression from infection to disease. Other conditions like diabetes mellitus, severe kidney disease, certain cancers, and the use of immunosuppressive medications also raise an individual’s susceptibility.
There is also a component of genetic susceptibility, which is separate from inheriting the disease itself. Researchers have identified specific genetic variations that can make an individual’s immune system less effective at fighting off the bacteria once exposed. For example, variations in genes like TYK2 or specific risk loci on chromosomes 3q23, 11p13, and 18q11 have been linked to a heightened risk of developing active TB. These genetic factors do not cause the disease but rather predispose the person to a poor outcome after they have already been infected by the external bacteria.
Latent and Active Disease
Tuberculosis infection exists in two distinct stages: latent TB infection (LTBI) and active TB disease. This distinction is based on the interaction between the bacteria and the host’s immune system.
Latent infection occurs when the immune system successfully walls off the bacteria, containing it in an inactive state. A person with LTBI has no symptoms, does not feel sick, and cannot transmit the bacteria to others. The bacteria are present but dormant, and the immune system’s containment prevents them from multiplying. However, the bacteria may remain viable for decades, posing a risk of future reactivation.
Active TB disease is the stage where the bacteria overcome the immune system’s defenses and begin to multiply rapidly, causing tissue damage. This stage is characterized by symptoms such as a chronic cough, unexplained weight loss, fever, and night sweats. A person with active pulmonary TB is contagious and can release the infectious droplet nuclei into the air, necessitating immediate treatment to prevent further spread.