The theory goes : plagues that are especially deadly must spread through the body extremely quickly. Otherwise, they give the immune system time for the B cells to formulate an antibody. Yet, if the plague spreads quickly, it has a short incubation period, and it means that hosts will die before spreading it. Ebola is thought to fit in this part of the ecology, and this is one reason why the virus is rare.
A virus that spread itself like the flu but also killed like ebola would be pushed by evolution away from these properties because it would kill off it's hosts too quickly.
Another factor is that some of the better viruses for evading the immune system (HIV) depend on being able to randomly recombine and change the pattern for their outer shells.
If you designed a virus that had a tough outer coating, targeted cells and receptors designed to kill the host, and had some kind of sophisticated clock mechanism to force a long incubation period, you would be forced to give it genes that would code for complex error correcting proteins so that each new generation of the virus would have a low chance of containing a mutation. This would in turn prevent it from evolving, allowing the immune system (and synthetic antibodies) to target it easily.
So, you'd have to deliberately make it able to adjust it's own outer coat randomly, but not any other components.
Such a virus is not something evolution is likely to ever create (because for one, it would extinct it's hosts, and for another, evolution doesn't work like this. Evolution as an algorithm finds the highest point on the NEAREST hill in the solution space, not the peak of a theoretical mountain that towers over the solution space)
Net result : with very sophisticated bioscience, such a person killer that had overlapping qualities could be created. However, you are correct that there is a reason you don't see them in nature.
Such a virus is not something evolution is likely to ever create (because for one, it would extinct it's hosts
This is a reason we would not expect to see many of them around at any given time. But a virus that can extinguish its hosts has no pressure to avoid evolving that way in the first place. Neither viruses nor evolution can predict the future, at all.
The FHI's mini advent calendar: counting down through the big five existential risks. The fourth one is an ancient risk, still with us today: pandemics and plagues.
Pandemics
Current understanding: high
Most worrying aspect: the past evidence points to a risky future
The deathrates from infectious diseases follow a power law with a very low exponent. In layman’s terms: there is a reasonable possibility for a plague with an absolutely huge casualty rate. We’ve had close calls in the past: the black death killed around half the population of Europe, while Spanish Influenza infected 27% of all humans and killed one in ten of those, mostly healthy young adults. All the characteristics of an ultimately deadly infection already exist in the wild: anything that combined the deadliness and incubation period of AIDS with the transmissibility of the common cold.
Moreover, we know that we are going to be seeing new diseases and new infections in the future: the only question is how deadly they will be. With modern global travel and transport, these diseases will spread far and wide. Against this, we have better communication and better trans-national institutions and cooperation – but these institutions could easily be overwhelmed, and countries aren’t nearly as well prepared as they need to be.