Although the virus that causes COVID-19 probably emerged from an animal source, it is thought to spread mainly from person-to-person via respiratory droplets produced when an infected person coughs or sneezes. People are thought to be most contagious when they are most symptomatic (the sickest). How easily a virus spreads from person-to-person can vary. Some viruses are highly contagious (like measles), while other viruses are less so. Another factor is whether the spread is sustained. The virus that causes COVID-19 seems to be spreading easily and sustainably in Hubei province and other parts of China. In the U.S., spread from person-to-person has occurred only among a few close contacts and has not spread any further to date.