Many people think of tuberculosis as being a disease from the past but the truth is far from it: Tuberculosis is mutating into dangerous new strains for which there is no known cure. One of the most frightening strains is XDR-TB, which stands for extensively drug-resistant TB. Unlike less virulent strains, XDR-TB does not respond to the antibiotics that are usually used to treat TB. The disease is virtually incurable and threatens to become a pandemic.

About 40,000 new cases of XDR-TB emerge every year, the World Health Organization estimates. Award-winning photojournalist James Nachtwey, who has chronicled the death and devastation the disease is bringing to many countries around the world, describes XDR-TB as "a merciless, man-eating predator lurking in the shadows."

He warns: "If it's not contained, the consequences could be dire."

Nachtwey, who has been covering humanitarian crises for more than 30 years, was awarded a TED prize in 2007 which gave him $100,000 and one wish to change the world. His wish centered on spreading awareness of this deadly form of TB and the images are borne out of Nachtwey's frustrations with the underreporting of what is potentially a global health crisis.