Nov 7 (Reuters) - International experts begin meeting on Thursday in Cape Town to discuss the problem of mounting resistance to tuberculosis drugs and the growing threat of drug-resistant forms of the disease.

AIDS activists have said African nations are failing to control TB, which is often linked to HIV and has dire implications for the war on AIDS.

In Africa, HIV is the single most important factor contributing to the increase in incidence of TB since 1990.

Here are some details about tuberculosus and its drug resistant variants:

* BACKGROUND:

-- A third of the estimated 40 million HIV-positive people worldwide are believed to be co-infected with TB and HIV. In South Africa, 61 percent of the roughly 250,000 people diagnosed with TB each year have HIV.

-- The emergence of extensively drug resistant TB (XDR TB), a strain virtually immune to traditional and modern antibiotics, has raised alarm bells since surfacing in South Africa's KwaZulu-Natal Province and neighbouring Lesotho in 2006, where it killed up to 85 percent of those infected.

* WHAT IS TUBERCULOSIS?

-- Tuberculosis, or TB, is a disease caused by bacteria called Mycobacterium tuberculosis. The bacteria can attack any part of the body, but they usually attack the lungs.

* HOW IS IT SPREAD?

-- TB is spread through the air from one person to another. Bacteria enter the air when a person with TB of the lungs or throat coughs or sneezes. People nearby may breathe in these bacteria and become infected.

-- In multidrug-resistant TB (MDR TB), the bacteria that cause the disease are resistant to the two most powerful anti-TB drugs, isoniazid and rifampicin. MDR TB is called extensively XDR TB if it also resists antibiotics in a class called fluoroquinolones, and at least one of three injectable second-line drugs -- capreomycin, kanamycin, and amikacin.

* SYMPTOMS:

-- Symptoms of active disease include a cough with thick, cloudy mucus, sometimes blood, for more than two weeks; fever, chills, and night sweats; fatigue and muscle weakness; weight loss; and sometimes shortness of breath and chest pain.

* NUMBERS:

-- Overall, one-third of the world's population is infected with the TB bacillus. An estimated 500,000 people globally have MDR TB, according to the World Health Organization. -- In 2005, 8.8 million people contracted TB and 1.6 million died.

-- Both the highest number of deaths and the highest mortality per capita are in the Africa Region. The TB epidemic in Africa grew rapidly during the 1990s, but this growth has been slowing each year, and incidence rates now appear to have stabilized or begun to fall. Sources: Reuters/WHO/Centers for Disease Control and Prevention. (Writing by David Cutler, editing by Mary Gabriel)

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