In this article published in Nature.com, a SARS-CoV-2 human challenge study found that of the healthy volunteers inoculated with the SARS-CoV-2 virus intranasally, only a little more than half became infected with mild symptoms, and the other half remained uninfected. Each person's own immunity plays a pivotal role, and immunity is linked to daily habits, stress levels and more things which we can actually control ourselves.