What is an epidemiological curve, and how do the “epi-curves” compare for covid-19 infections for China, Italy, Germany, Singapore, and the United States?

What is an epidemiological curve, and how do the “epi-curves” compare for covid-19 infections for China, Italy, Germany, Singapore, and the United States? What were key developments in assessing and containing the spread of covid-19 in China, Italy, Germany, Singapore, and the United States?