Different viruses are transmitted differently -- think HIV vs. the common cold. We are always learning new things about viruses, but in this case the virus was new that we didn't know as much about the most prevalent transmission methods. There are specific papers and sites such as the below discussing the research of this. https://pubmed.ncbi.nlm.nih.gov/33200716/ https://www.cdc.gov/coronavirus/2019-ncov/science/science-briefs/sars-cov-2-transmission.html