A growing number of American states are launching smartphone systems designed to warn people of possible exposure to COVID-19. The release of the apps comes as parts of the United States continue to struggle with the spread of COVID-19 and after President Trump announced that he had become infected. States and local communities set their own restrictions and guidelines for citizens to follow to reduce spread of the virus. The federal government did not build a nationwide app, so states have worked with developers to create their own. The tool could be a big help to health officials trying to deal with a fast-spreading disease that many people do not even know they have. Many infected individuals show few or no signs of sickness. If a person learns they were near someone who later got infected, they could immediately isolate or seek treatment.