TB or Mycobacterium tuberculosis has been killing people for thousands of years. Tissue samples from Egyptian mummies over 4000 years old show signs of being infected with the disease.

Some estimate that TB was responsible for around 20% of all deaths in England and Wales in the 17th and 18th centuries. In the 19th and 20th centuries there has been a steady decline in deaths from the disease in industrialised countries. This was given a boost by the development of penicillin and other antibiotics in he last 50 years.

But the decline began to level out in he 1980s and since then the incidence of the disease has started to increase again. Some scientists have said the number of people around the world infected with TB has reached a 10-year high.

What is TB?

TB is a disease which usually attacks the lungs, but it can affect almost any part of the body. A person with TB does not necessarily feel ill but the symptoms can include a cough that will not go away, feeling tired, weight loss, loss of appetite, fever, night seats and coughing up blood.

Like the common cold, TB is spread through the air after infected people cough or sneeze.

There is a difference between being infected with TB and having the disease. Many people infected with the TB bacteria do not develop the disease, as their body's defences protect them. Neither can they pass the disease on. But TB can lie dormant in the body for many years and strike when the immune system is weak.

Why is TB making a resurgence now?

One factor in the rising TB trend in both the developed and the developing world is HIV infection, which weakens the immune system. One third of deaths of those who are HIV-positive are TB related. Those with HIV are 100 times more likely to develop TB than other members of the population. Other people who are at risk from the disease include those with diabetes, the malnourished, alcoholics, and IV drug users.

Another aspect in the resurgence of the disease is the development of drug resistant strains which now affect up to 50 million people. These strains can be created by bad medical practice such as over-prescribing antibiotics or patients not taking the drugs long enough to get rid of the disease. Instead this encourages the bacteria to become tougher.

Treating patients with drug resistant TB is beyond the pocket of many developing countries. The cost of treatment can rise from $2000 per patient with non-resistant TB to $250,000 for multi-drug resistant TB.

The multi drug resistant strains are often fatal and have mortality rates that are comparable with those which existed before the development of antibiotics.

Some experts also blame lax public health procedures at immigration control for the rise in TB in developed countries.

Visitors arriving from less developed countries where TB is more prevalent may get little medical attention even if they admit to having the disease. Among black Africans in Britain TB rose by over 100% between the end of the 1980s and early 1990s.

How much of a threat is the disease?

According to the WHO, TB infection is currently spreading at the rate of one person per second. It kills more young people and adults than any other infectious disease and is the world's biggest killer of women. In 1993 the WHO declared TB "a global health emergency".

Researchers have calculated that 8-10 million people catch the disease every year, with three million dying from it. It causes more deaths world-wide than Aids and malaria combined.

The WHO predicts that by 2020 nearly one billion people will be newly infected with TB, of them 70m will die. TB blackspots include eastern Europe with 250,000 cases a year, south east Asia, three million cases a year and sub-Saharan Africa with two million cases a year.