An infection caused by the bacteria Mycobacterium tuberculosis. The bacteria are usually transmitted by inhaling or ingesting tiny air borne droplets, from coughing and sneezing or being in the breathing zone of a person with active TB. Primary (early) infection is often asymptomatic but the bacteria remain latent in the lung cells. A secondary infection can develop if the CD4 cell count falls below 500 and the bacteria is reactivated. The bacteria cause lesions, most often in the lungs, but it may spread to other organs. Early symptoms of secondary TB are cough, fever, night sweats and weight loss. If left untreated more severe symptoms, such as haemorrhage and pneumonia can develop; TB is not common in Australia but it is a risk when travelling to countries where the prevalence is high. Antibiotics are used to treat active TB and to prevent a reactivation of latent infection.