TB back in the news again in the U.S.

After years of hearing very little news on tuberculosis (TB), these days it seems to appear in the news on an almost weekly basis.

The latest outbreak is again in the United States and concerns four employees at the state medical examiner's office in Boston.

All four have apparently tested positive for tuberculosis but it remains unclear how they contracted it.

The quartet reportedly have a latent form of tuberculosis, but none has an active case of the disease and the authorities say there is no cause for alarm.

Business at the examiner's office, where around 60 people work, is expected to continue as usual.

Testing for TB was carried out there last week but it is unclear whether this was routine practice or because a positive case had been identified.

Health officials have visited the office to discuss the outbreak with the staff.

Tuberculosis is a contagious disease. Like the common cold, it spreads through the air. Only people who are sick with TB in their lungs are infectious. When infectious people cough, sneeze, talk or spit, they propel TB germs, known as bacilli, into the air. A person needs only to inhale a small number of these to be infected.

Left untreated, each person with active TB disease will infect on average between 10 and 15 people every year. But people infected with TB bacilli will not necessarily become sick with the disease. The immune system "walls off" the TB bacilli which, protected by a thick waxy coat, can lie dormant for years. When someone's immune system is weakened, the chances of becoming sick are greater.

• Someone in the world is newly infected with TB bacilli every second.

• Overall, one-third of the world's population is currently infected with the TB bacillus.

• 5-10% of people who are infected with TB bacilli (but who are not infected with HIV) become sick or infectious at some time during their life. People with HIV and TB infection are much more likely to develop TB.

The World Health Organization (WHO) estimates that the largest number of new TB cases in 2005 occurred in the South-East Asia Region, which accounted for 34% of incident cases globally. However, the estimated incidence rate in sub-Saharan Africa is nearly twice that of the South-East Asia Region, at nearly 350 cases per 100,000 population.

It is estimated that 1.6 million deaths resulted from TB in 2005. 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.

In 2005, estimated per capita TB incidence was stable or falling in all six WHO regions. However, the slow decline in incidence rates per capita is offset by population growth. Consequently, the number of new cases arising each year is still increasing globally and in the WHO regions of Africa, the Eastern Mediterranean and South-East Asia.