In 1972 news of a long-term medical experiment, conducted by the Public Health Service since the early 1930s on black sharecroppers from Alabama, became public. The Tuskegee Experiment, as it's called, gave medical "treatment" to African-American men with syphilis. The true meaning of the study was kept from the men. Its goal was to conduct autopsies on them after they died, to see the results of damages caused by this sexually transmitted disease. They were given free treatment for something called "bad blood." Doctors in the experiment did not try to cure the syphilis, even after penicillin was discovered.
You can read more about it here.
Ten years before that I read an article about a propaganda movie from the Soviet Union, which had something of the same idea. At the time It was derided as an outrageously untrue portrait of American society and science. The 1954 article about the movie, Silvery Dust, in LIfe, was in a stack of old magazines I found about 1961 in the library of my junior high school. At the time, like Life, I thought a plot involving black men for a medical experiment (in this case, being falsely accused of rape, and then used as guinea pigs for a killer radioactive dust), could not possibly happen in America. We were a lot more patriotic in those days—jingoistic. really—and more trusting of our government and its motives.
Life treats the Soviet movie strictly as hate-filled anti-American propaganda. On the other hand, unknown at the time to Life or the film's Russian producers, some of its ideas were being carried out in the U.S. by a government department.
The USSR had a history of murdering its citizens for various reasons, mostly political, but the U.S. was supposed to be above that. At least we were taught that in school. Something they didn't tell us at the time was what the effects of open air testing of nuclear devices was doing to citizens downwind. The Atomic Energy Commission is quoted, "Fallout does not constitute a serious hazard outside the test site." The AEC was well aware of the results from Hiroshima and Nagasaki, and knew that statement was a lie. Sometimes what is done in the name of national security causes some folks much less security. In this case it was the people of Southern Utah, who were exposed. The bombs weren't set off until the prevailing winds blew in the right direction, over a "low yield" segment of the population, Nevada and Utah.
These are the sorts of things that make citizens fear and mistrust their government, the basis for conspiracy theories. There are plenty of conspiracy theories to go around. The U.S. government figures into most of them. While I'm a skeptic on grand, far-reaching conspiracies, it chills me to think what if? What if conspiracy theorists of the past had screamed that the government was conducting inhumane medical experiments on African-American males, or that open air testing was done to make Southern Utah residents guinea pigs for radiation testing? Would anyone have believed them?