Https://theconversation.com/what-is-a-virus-how-do-they-spread-how-do-they-make-us-sick-133437 Viruses are the most common biological entities on Earth. https://www.skypat.no/pathology/2022/02/13/what-is-a-virus-how-do-they-spread-how-do-they-make-us-sick-hentet-fra-the-conversation/