FACTBOX: Tuberculosis - a leading killer disease

CANCUN, Mexico (Reuters) - More than 2 billion people, or a third of the world's total population, are infected with Mycobacterium tuberculosis, the bacteria that causes tuberculosis.

Tuberculosis is now the world's seventh-leading cause of death. It killed 1.8 million people worldwide last year, up from 1.77 million in 2007. It is one of three primary diseases that are closely linked to poverty, the other two being AIDS and malaria.

Here are some key facts about tuberculosis:

* Tuberculosis is spread easily through the air. When infectious people cough, sneeze, talk or spit, they expel the bacteria and just a small amount is enough for transmission. Someone in the world is newly infected with TB every second.

* Nearly all TB infections are latent, with carriers showing no symptoms and they are not infectious. However, one in 10 will become sick with active TB in his or her lifetime due primarily to a weakened immune system.

* Of the 1.8 million deaths in 2008, or 4,930 deaths a day, half a million were AIDS patients. TB affects mostly young adults in their most productive years. The vast majority of TB deaths are in the developing world, with more than half occurring in Asia.

* The World Health Organization estimates that 9.4 million people developed active TB in 2008, up from 9.27 million in 2007 and 9.24 million in 2006. Among the 15 countries with the highest TB incidence rates in 2007, 13 were in Africa, while half of all new cases were in six Asian countries -- Bangladesh, China, India, Indonesia, Pakistan and the Philippines.

* TB does not figure among the top 10 causes of death in developed nations but is the seventh-highest cause of mortality in poor countries.

The average TB patient loses three to four months of work and up to 30 percent of yearly household earnings. The World Bank estimates that the disease diminishes 4 percent to 7 percent of GDP in some of the worst-affected countries.

* Drug-resistant TB is caused by inconsistent or partial treatment, when patients do not take all of their medicines regularly for the required period because they start to feel better, because doctors and health workers prescribe the wrong treatment regimens, or because the drug supply is unreliable.

* A particularly dangerous form of TB is multidrug-resistant TB (MDR-TB), which is TB that resists at least isoniazid and rifampicin, the two most powerful anti-TB drugs.

* Rates of MDR-TB are high in some countries, especially in the former Soviet Union, and threaten TB control efforts. MDR-TB is present in virtually all countries surveyed by the World Health Organization.

* Extensively drug-resistant TB, or XDR-TB, is a relatively rare type of TB. XDR-TB is defined as TB that is resistant to isoniazid and rifampin as well as any fluoroquinolone and at least one of three injectable second-line drugs. Between 35 percent and 50 percent of patients with this form of TB die.

Sources: WHO and U.S. Centers for Disease Prevention and Control

(Reporting by Tan Ee Lyn; Editing by Julie Steenhuysen)