A healthy immune system is important at all times, but especially so in these days of the novel coronavirus and heightened awareness about the spread of germs. You can support a healthy immune system by eating well, exercising and getting plenty of sleep. Supplements with vitamins that support the immune system, such as vitamin D, may help, too. Sunlight remains a key source of vitamin D, but because people are less able to get outside, supplements may be increasingly beneficial. This is especially true for those who live in the northern hemisphere. As an immune- system support, vitamin D has the science to back up its potential. In 2017, the British Medical Journal published a meta-analysis of 25 studies in which vitamin D showed the potential to protect against acute respiratory tract infections, especially among individuals who were also deficient in vitamin D. In addition, Dr. Tom Frieden, former director of the Centers for Disease Control and Prevention, said in an interview w...