How do scientists learn about new viruses?

How do scientists learn about new viruses?

Fundamentally, we know about them because they cause disease. And often they only cause disease in the one species. So sometimes it's not possible to do a Koch's Postulate. Like I will inject a mouse and get the disease, because they don't infect the mouse. Or if they did infect the mouse, the mechanisms are completely different, and then the virus is very low. So very often, and the case of HIV is a good example, it doesn't affect anything virtually but man bar a small affect on the chimp species.