Great question, and a very popular one with news of virus outbreaks happening across the world recently.
In a recent Washington Post article that attempts to tackle this very qeustion, Patrick Sanchez, a healthcare professional in Sacramento, suggests that face masks are most effective for those who are already sick, rather than someone who is trying to avoid catching something themselves. Of course, if you are a medical professional, it’s best to wear a face mask whenever you’re interacting with patients.
Amy Shah, a double board-ceritifed medical doctor in allergy and immunology, is also quoted in this article stating that frequently washing your hands for at least 20 seconds with water and antibacterial soap and wearing disposable gloves are other precautions that are likely even more effective for preventing viruses, like the Coronavirus.