Scientists offered more evidence Wednesday that the coronavirus is spread by seemingly healthy people who show no clear symptoms, and the federal government issued new guidance warning that anyone exposed to the disease can be considered a carrier. A study by researchers in Singapore became the latest to estimate that somewhere around 10% of new infections may be sparked by people who carry the virus but have not yet suffered its flu-like symptoms. In response to that study and others, the Centers for Disease Control and Prevention changed how it defined the risk of infection for Americans. The agency’s new guidance targeted people who have no symptoms but were exposed to others with known or suspected infections. It essentially says that anyone may be a carrier, whether that person has symptoms or not.