Why are some people with coronavirus asymptomatic — and why are they so contagious?

Just how contagious is someone who has contracted COVID-19 — yet displays no symptoms — and what makes them asymptomatic while other people suffer so terribly with the disease? This study, published in the medical journal JAMA Internal Medicine this week, provides one theory for the first question. It isolated 303 patients with COVID-19 in a treatment center in South Korea. Of those, 110 (36%) were asymptomatic and 21 (19%) developed symptoms during isolation. “The potential for transmission from asymptomatic people has been cited as an important factor in controlling the spread of COVID-19, but there is limited information about the clinical course and viral loads of asymptomatic people...