All cases of TB are passed from person to person via droplets. When someone with TB infection coughs, sneezes, or talks, tiny droplets of saliva or mucus are expelled into the air, which could be inhaled by another person.
Once infectious particles reach the alveoli, small sacs in your lungs, another cell called the macrophage engulfs the TB bacteria.
Then the bacteria are transmitted to your lymph system and bloodstream and spread to other organs.
The bacteria further multiply in organs that have high oxygen pressures, such as the upper lobes of your lungs, your kidneys, bone marrow, and meninges—the membranelike coverings of your brain and spinal cord.
When the bacteria cause clinically detectable disease, you have TB.
People who have inhaled the TB bacteria, but in whom the disease is controlled are referred to as infected. They have no symptoms, frequently have a positive skin test, yet cannot transmit the disease to others.
Risk factors for TB include the following:
Low socioeconomic status
Crowded living conditions
Diseases that weaken the immune system
Migration from a country with a high number of cases
Health care workers