This goes against the belief of many doctors that patients who have two attacks of TB have simply failed to eradicate the bug on the first occasion.

And the fact that natural infection does not give you immunity will make it harder to develop more effective vaccines against the bacteria.

South African researchers reported in Thursday's New England Journal of Medicine that tests on people whose tuberculosis had come back found that the new bacteria had a different genetic structure to the original bacteria.

This meant it was not simply that the treatment given for the first infection had failed, but that a completely different bug had taken over.

They looked at the TB bacteria from both infections of 16 patients - in 12 cases, or 75%, the first and second bacteria had different DNA fingerprints.

Vaccine for former patients

Dr Paul Fine, from the London School of Hygeine and Tropical Medicine said that this meant that people who had recovered once from TB should be given vaccinations wherever possible.

He added: "If natural infection does not confer protective immunity, the development of improved vaccines against tuberculosis will be especially challenging and the need for better understanding of host defences against tuberculosis particularly critical."

He said the study was an "important clue" for those trying to eradicate TB.

The World Health Organisation is currently running a massive worldwide programme aimed at drastically reducing the number of TB deaths over the next decade.

Currently, 3.3m are thought to suffer from the disease, which attacks the lungs.

Many carry it, but few suffer

Bacteria are carried by many people, but often this does not result in disease developing.

Even in those who do contract the disease, there is often a long pause between infection and illness, and scientists do not know why this is.

TB is particularly dangerous for people whose immune systems are not working well due to either other infections, such as HIV, or general poor nutrition and unhealthy lifestyle.

A rise in TB cases reported in Africa has been attributed to soaring HIV infection rates.

Treatment for TB involves a long course of drugs, and in developing countries it is often difficult to ensure a patient fully completes the course, as the tablets must be taken long after he or she feels better, to ensure that all the TB bacteria are killed.