Pathogenesis

Tuberculosis (TB) is a contagious disease. Like the common cold, it spreads through the air. Only people who are sick with TB in their lungs are infectious. When infectious people cough, sneeze, talk or spit, they propel TB germs, known as bacilli, into the air. A person needs only to inhale a small number of these to be infected.

TB is spread by aerosol droplets expelled by people with active TB disease of the lungs when they cough, sneeze, speak, or spit. Each droplet is 5 µm in diameter and contains 1 to 3 bacilli. Close contacts (people with prolonged, frequent, or intense contact) are at highest risk of becoming infected (typically a 22% infection rate). A person with untreated, active tuberculosis can infect an estimated 20 other people per year. Others at risk include foreign-born from areas where TB is common, immunocompromised patients (eg. HIV/AIDS), residents and employees of high-risk congregate settings, health care workers who serve high-risk clients, medically underserved, low-income populations, high-risk racial or ethnic minority populations, children exposed to adults in high-risk categories, and people who inject illicit drugs.

Transmission can only occur from people with active TB disease (not latent TB infection).

The probability of transmission depends upon infectiousness of the person with TB (quantity expelled), environment of exposure, duration of exposure, and virulence of the organism.

The chain of transmission can be stopped by isolating patients with active disease and starting effective anti-tuberculous therapy.