There is really no conspiracy here at all. It has happened. MK-ULTRA was an operation the CIA used to attempt to control people's minds with LSD back in the '50s. Or how about the Tuskegee experiments? My guess is the people that had syphillis would rather have known they had the disease and had it treated.
Here is an interesting wiki entry that gives a brief summary of "proven conspiracy theories."
http://en.wikipedia.org/wiki/List_of_proven_conspiracies
