In 2015, Chicago’s public health department identified three cases of invasive meningococcal disease , a highly dangerous pathogen that can kill within hours . To determine whether these cases signaled a larger outbreak, the department—which one of us, Dr. Julie Morita, led at the time—needed to know not just total cases, but the total population and demographics of affected areas. The team turned to a pillar of our nation’s public health system: the U.S. Census .

The data showed that men who have sex with men—including gay and bisexual men—among the city’s Black population were disproportionately affected. That finding drove the health department’s response, which included working with community partners to surge thousands of free vaccines into neighborhoods across Chicago. Instead of a larger and far deadlier outbreak, the outbreak ended with 11 cases in Chicago and one death .

Census data informs every aspect of our nation’s public health system.…