Sensors on asthma inhalers generate real-time maps of environmental dangers to help patients, physicians and disease detectives in Louisville, Kentucky. Street knowledge was also crucial in a historic medical breakthrough: John Snow’s mapping of cholera fatalities in 19th century London. In West Oakland, California, citizens confront air pollution and rising asthma rates by collecting traffic data. Local ordinances are changed and everyone breathes easier. Can apps and maps combat globalized diseases in a warming world? Stories of citizen science fighting mosquito-borne diseases with apps and crowd-sourced data in Barcelona, Houston and New Orleans. In Kenya, Medic Mobile develops smart but low-cost software to give simple phones powerful capabilities to help community health workers improve maternal and child health.