A true one, this.
Just shy of ten years ago, in the aftermath of 9/11, I wrote my dissertation on conspiracy theories. Some of them turn out to be true, or at least have a semblance of truth to them.

The major one which springs to mind is The Tuskegee Experiment. Basically the theory was that black people were rounded up in the 1930s, injected with syphilis, and then the effects of the disease were monitored whilst the victims were denied proper treatment.

And oddly enough the theory turned out to be pretty accurate, except for one minor detail. The victims weren't given syphilis - they had already contracted it. However they were denied treatment to the extent that if they requested penicillin they were instead given a placebo, and they were also subjected to painful experiments such as spinal taps. Oh, and when other family members contracted the disease they were denied treatment too.

It took 40 years for the full extent of what was going on to come to light. That's not 40 years after the event took place, but 40 years of testing... so up to the 1970s this was still going on.
(Captain Skippy pooed in a bag and left it on your doorstep, Mon 5 Dec 2011, 6:14, Reply)