COVID-19 individuals experience symptoms ranging from none, to mild, to severe. Research shows even a low viral load may lead a healthy individual to become infected with the virus.

The minimum infectious dose of COVID-19 is unknown so far, but researchers suspect it is low. Exposure to higher doses, or frequent low doses, of coronavirus may lead to worse symptoms and/or health outcomes. COVID-19 patients with severe symptoms tended to have higher viral loads.

Summary StatNews
Research from  National Center for Biotechnology Information & The Lancet