Narrator: This is Science Today. Researchers at the University of California, San Francisco, have shed light on a longstanding mystery about how the tuberculosis microbe can outsmart a healthy immune system and cause disease. Dr. Joel Ernst is an infectious disease specialist.

Ernst: Mycobacterium tuberculosis, which is the bacteria that cause TB, has devised means of infecting a tremendous number of people without killing them quickly.

Narrator: That's because this crafty bacterium, which actually lodges inside killer immune cells called macrophages, disrupts the final stage of the macrophage's biochemical battle by blocking signals from interferon gamma - a protein needed to stimulate macrophages into action.

Ernst: I think the importance of what we've done so far is to simply reinforce and to show that a vaccine strategy directed at Mycobacterium tuberculosis is not likely to be successful if the only effect of the vaccine is to induce other cells in the immune system to produce interferon gamma when they recognize Mycobacterial tuberculosis.

Narrator: For Science Today, I'm Larissa Branin.