More than 300 cases of the highly infectious disease, which is spread by airborne droplets and kills 98% of those infected within about two weeks, have been identified in South Africa.

But doctors believe there have been hundreds, possibly thousands, more and the numbers are growing among the millions of people with HIV, who are particularly vulnerable to the disease. Their fear is that patients with XDR-TB, told that there is little that can be done for them, will leave the isolation wards and go home to die. But while they are still walking around they risk spreading the infection.