A scanning electron micrograph of Mycobacterium tuberculosis bacteria, which cause TB. BSIP/Universal Images Group via/Getty Images BSIP/Universal Images Group via/Getty Images TB or not TB? That has been the question for nearly 150 years, when Dr. Robert Koch first identified the rod-shaped bacterium that causes tuberculosis. This serious respiratory illness is currently the world's deadliest infectious disease, killing more than a million people a year. The most common test to determine if someone has tuberculosis hasn't really changed since the late 1800s. The process relies on phlegm. "It's a nasty substance," says Adithya Cattamanchi, a pulmonologist at UC Irvine. "No…