It is a term used to refer to the curve in the projected number of people who will contract a pathogen over a period of time. If models are showing that the pathogen is expected to infect people quickly, the curve upward is sharp. So public health officials look for ways to slow the spread the pathogen, or flatten that curve, to prevent hospital systems from becoming overwhelmed with patients.
Deeper dive
The term became part of the American lexicon during the early months of the COVID-19 pandemic as government leaders and public health officials sought to explain why strict social distancing measures were being enacted to stop the spread of SARS-CoV-19.
Before vaccines became widely available in early 2021, the only option to slow the transmission rate, and flatten the infection curve, was through collective social distancing, which meant avoiding other people whenever possible.
When someone is infected with COVID-19, that individual can spread it to multiple people and if nothing is done to slow its transmission it could overwhelm hospitals. As of mid-2022, rather than emphasizing social distancing, public health officials are emphasizing testing, masking in indoor public places, vaccination, and quarantine for five days if infected, to prevent spread of COVID-19.