When the first U.S. case of the new coronavirus, COVID-19, was confirmed on Jan. 21, it appeared to be contained — the patient was a man living near Seattle who had recently returned from Wuhan, China and immediately isolated himself, and the local health department had conducted contact tracing to make sure that anyone he had encountered was now quarantining at home.
But as more people returned to the U.S. from China, and then Italy, the site of the second major outbreak, the number of cases started to tick upwards. And due to the nature of the virus — which is highly contagious but can present without symptoms for many people — it began to spread in communities where there were no known cases and no testing measures in place.
From there, the number of cases soared — up to 1,000 by March 26, and to 1 million by April 28.
That spread can be seen in the above graphic, which shows how the virus circulated through the country over the last few months.