Patient Zero.
Sounds like a movie...
Actually, for this illness, it was a book and not a movie, released in 1974. This disease has been around awhile.
https://www.amazon.com/Fever-hunt-new-killer-virus/dp/0883490129
“The Hot Zone”
Sounds like a book. I read it years ago and learned about Marburg, Lassa, and Ebola. Scary diseases. The “good” news is they are so deadly that they burn out rather quickly.