When President Donald Trump was diagnosed with COVID-19, his pill arsenal included Vitamin D and zinc. And in an Instagram chat with actress Jennifer Garner in September, infectious diseases expert Anthony Fauci touted vitamins C and D as ways that might generally boost the immune system. “If you’re deficient in vitamin D,” he noted, “that does have an impact on your susceptibility to infection. I would not mind recommending, and I do it myself, taking vitamin D supplements.”