Hank: Few bacteria take us down like mycobacterium tuberculosis. TB kills over 1 million people every year making it the world's second deadliest infectious disease. But it wasn't always that way.

TB bacteria's genetic ancestors were less deadly, which scientists have used to study how the disease became this huge problem. Researchers looked at the TB bacteria's ancestors and identified the strains that survived the longest in mice. It turns out, the more persistent ones were resistant to the effects of a molecule called "nitric oxide."

Typically, our immune cells make nitric oxide to get rid of bacteria; it's super-reactive, so when it crosses into the bacteria's cells, it damages their DNA. But these persistent forms of the bacteria had two key mutations in genes that are involved in bringing stuff into the cell - stuff like nitric oxide. So, their outsides are harder to penetrate, and they are harder to kill, which explains why our natural defenses often are not enough to tackle TB, and gives us a potential avenue for future treatments.

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