As US leaders work to control the spread of coronavirus, researchers across the globe are working to answer the mysteries that remain around infections. One of those mysteries: why the experience can be so different from person to person. One expert says the answer may involve looking at previous vaccines individuals have had. "When we looked in the setting of Covid disease, we found that people who had prior vaccinations with a variety of vaccines -- for pneumococcus, influenza, hepatitis and others -- appeared to have a lower risk of getting Covid disease," Dr. Andrew Badley, an infectious disease specialist at Mayo Clinic, told CNN's Anderson Cooper on Monday night. It's what immunologists call immune training: how your immune system creates an effective response to fight off infections, Badley says.