Bird flu is not the path, but it would be easy to do.
It is so easy it happens in nature. The 1918 Flu killed 50,000,000. From healthy to dead in twelve hours, no treatment would work. By the time symptoms show, the damage has been done.
There is a real reason millions of chickens are killed, it is just a matter of time.
Second problem, of our making, disinfectant soaps, and drugs in the sewer. Kills some, the survivors become resistant. The last cases of flesh eating bacteria, from rivers. It used to only show up in hospitals.
Third problem, clean children living in filtered air, do not develop resistance.
There are things worse than you can think of. Possible no survivors, no one immune. The 1918 Flu killed young people, old people who had other flus were immune.
Here at the bottom of the flyway, West Nile, Equine Encefalitus, are becoming common, and with warming, Dengue is moving north. Hanta virus in the west does not kill its mouse host, just humans who come in contact with feces.
Where treatment is no longer possible only containment works, like Andromida Strain, kill them and burn the bodies.
It seems we killed off the good bacteria that were keeping the bad ones down.
Welcome to evolution!