Viruses generally weaken over time due to better host adaptation to hide from the immune response. If Unicron came from someone with HIV that caused several years of adaptation in a few months, then that can be why it's less severe. When they first jump into humans, our bodies recognize them quickly as foreign, and it often goes overboard with the immune response, so higher fatality rates. SARS-CoV-2 was quite well adapted to humans from the beginning, hence why it's nowhere near as bad as SARS, but it can still adapt more, which it has done.
For those interested, you should take a look into OC43 from the late 1800s. A common cold coronavirus now, but it looks like it caused something similar to SARS-CoV-2 when it first came along. It even had something akin to "Long COVID" too.