TB is spread when people who have active TB sneeze, cough, or speak and propel their germs into the air. All it takes is for a healthy person to inhale one infectious particle to become infected. People with active TB, ie those who have symptoms – can infect between 10 and 15 people in one year.
 

To breathe in TB germs, however, you need to have very close, day-to-day, contact with an infected person. That's why most people get TB germs from someone they spend a lot of time with, like a family member, friend or close co-worker.
 

People with a healthy immune system are not likely to get the disease. Only 1 in 10 healthy people who are infected with the bacteria will develop TB. The risk, however, is greater in babies and young children, the elderly, and people who are malnourished. When someone’s immune system is weakened or suppressed, including organ transplant recipients, cancer patients receiving chemotherapy and people with HIV infection or full-blown AIDS, they are more vulnerable and their chances of catching TB are greater.
 

There are people who remain infected but have no symptoms. The bacteria can lie dormant within for many years. The organisms do not grow, but they are still alive. As soon as an opportunity crops up – in old age, or when a person is weakened by another disease, for example – the bacteria reawaken and cause tuberculosis.