Viruses don't diffuse through the air like smoke. Viruses are contained within the aerosolised droplets of your bodily fluids that are ejected through sneezing, coughing, blowing the nose. The survival of the virus outside of the body exposed to the elements is limited. Viruses have limited lifespans and can only reproduce within the medium of the bodily fluid. Every virus has different characteristics, and may have a longer or shorter lifespan. These characteristics are more or less universal for viruses though. It's not impossible for them to travel longer distances through the air. For the most part they are limited to the initial propulsion from the body. A better analogy would be spraying an aerosol can and watching how the droplets settle.