You seem to have two questions. The first is if an organism or
virus could be engineered which would kill 100% of those infected.
The second is if it is possible that every last person on Earth
could become infected with such an agent.
The answer to the first question is yes. There are diseases,
such as rabies (which is viral) which will always kill the
host, unless treated promptly. HIV kills virtually 100% of those
infected, whether or not treatment is received.
The answer to the second is probably not. People are so numerous
and so widespread that it would require a herculean effort to expose
every last human to some infectious agent.
In the end, humanity has weathered many a devastating plague, but
we are still here. It could be argued that humanity has never
weathered a plague specifically engineered to cause us to become
extinct (although genocide by disease has been attempted, such as
in the case of US attempts to eradicate native American populations
by the distribution of blankets taken from smallpox wards -- notice
that although native American populations were devastated, some
did survive.) I would counter that, if the ability to fight using
disease has improved, so has humanity's ability to fight disease
itself.
I have heard the figure 60 million bandied about as the number of
casualties which could be inflicted on the US in one month by a
well-coordinated germ attack (an old roommate's father who retired
from Ft. Deitrich some years ag.)o That is a lot of people to
be sickened or killed, but it is less than a well-coordinated
thermonuclear offensive would kill.
Grim grim grim. What is the defense from such things?
