It also depends on the density and mobility of your host population.
Ebola kills too quickly for hosts to move around and spread it, but that's in small villages in the jungle. What if there's just enough time for the host to take a crowded train and attend a Presidential campaign rally with tens of thousands of other people before feeling too sick? This might be a better strategy for an ambitious virus in the post-Covid world than a slowly escalating illness that just makes people call in sick and stay home.
It's a bit of a shame that Plague Inc does not support re-infection. Once someone had the disease, they are immune for life.
And the only goal the game really supports is killing everyone, instead of eg going for the largest sustainable population of your organism. (Think more like one of the bugs that cause the common cold, and less like the black death.)
It also doesn’t seem very realistic that the virus can infect everybody and then evolve to start killing them, and somehow the past infections all get the updated orders to kill, haha. Gameplay concessions I guess.
One of these days, someone's going to genetically engineer a virus that spreads easily to infect as many people as possible, but stay dormant for a few years while in this phase, then suddenly it'll turn lethal. Someone from the future might use a time machine to try to come back and gain information about the virus, and then even try to stop its spread once he finds the rogue researcher releasing it at airports, but then will tragically find out that you can't change the timeline, even if you can travel back in time.
Even then it seems like a crapshoot whether or not it works, because somehow if you become highly infective but otherwise are flying under the radar you'll still be detected and suddenly all governments start closing up shop... or at least that was the case in Pandemic II. I can't speak to any of the clones.
A sample of the virus taken from the worker was 100% lethal in ferrets, though it spread inefficiently and does not appear to be continuing to spread.