The tool shows just how dangerous a group of 50 or more people is for disease transmission in the Bay Area, at present.
In Los Angeles County, in groups of 50 people, you'd have an 87-percent chance of finding an infected person right now.
In New York City, where case counts have slowed considerably since the spring, you're looking at a 25-percent chance in groups of 50, and in Chicago it's currently a 50-percent chance.