The extremely virulent form of tuberculosis that killed 52 out of 53 people at a hospital in KwaZulu-Natal, is just the "tip of the iceberg" in southern Africa, scientists have warned. What makes this strain of TB so lethal is that unlike normal TB, XDR TB can infect even the healthiest of people. But healthy people are more likely to be able to fight off the disease, while those who have compromised immune systems, will die within a month.