Wouldn't it be impossible to keep a secret like that?

It is comforting to imagine that our government is incompetent at keeping secrets,
but secrecy is really the lifeblood of our government's intelligence empire.  The secrecy
is a thing entailing detailed procedures, strict compartmentalization and severe penalties.

The question brings to mind a book, "Nation of Secrets: The Threat to Democracy
 and the American Way of Life", by Professor Ted Gup, of my alma mater.  This book
details the growth of government secrecy and the dangers entailed.

As the public increasingly loses hope of such a luxury as "privacy", our government
grows ever-more secretive.  There is probably a great deal of information that is stamped
as "classified" when it is completely tame and harmless, much less a bombshell  like
AIDS origin.  As for the penalties for spilling secrets- just ask Ed Snowden,
the NSA whistleblower- What are the rewards for attempting to do a good deed?
How effective are the whistleblower "protections"?

Why were others in the NSA afraid to speak, when they knew the same?
Because they knew, as Snowden did, what would happen to them if they tried
to speak out.

A person can be threatened with career ruination, or with prison.  If the issue is explosive|
enough, they can be threatened for their safety and lives.   The taking of their lives would
also be a secret in the name of national security.

National security secrets are important to national security in large part because it would
likely cause World War III if other nations could be certain of exactly what we have
done, much less the rioting in the streets and mobs of peasants with pitchforks if our own
people were aware of everything that their government does.

A good example of a well-kept secret was the Manhattan Project, to build the atomic bomb
during World War II.  A woman whose father had worked on the Manhattan Project told
me that not even the men who worked on that project knew when, where or even if the
weapon that they had developed would be used, much less the public.

What was true for the nuclear Manhattan Project would almost certainly be true for
the biological Manhattan Project.