As Covid-19 spreads, doctors are learning more about why some patients get very sick, and why others only get mildly ill. Some of the people most at risk for severe illness have underlying conditions that affect their lungs. Older people are also at a higher risk. But certain factors, when combined with age, create a powder keg for the effects of the disease. Both smoking and obesity are conditions that can lead to fatal results in Covid-19 patients. Bloomberg Senior Editor Jason Gale explains how these conditions have made the coronavirus more lethal in some countries.