I have two favorite conspiracy theories. They’re both plausible and ridiculous at the same time. I suppose I've always enjoyed a good paradox.
Periodically, someone will muse that scientists have discovered the cure for cancer/HIV/whatever-your-favorite-disease-is, but that they’re suppressing the information because it’s more profitable to treat those diseases than it is to cure them. The logic is there: an ounce of prevention is worth a pound of cure.
The part that is missing is the rationale for sitting on important data. Do you have any idea how famous you would become if you discovered the cure for cancer? It is every scientist’s dream to solve an important puzzle, make as meaningful a contribution to humanity and become as famous as Salk (who created the first cheap vaccine for polio), Flemming (who first purified penicillin) or Pasteur (who discovered an immunization against smallpox, eventually leading to the eradication of the disease). If I discovered the cure for cancer and then withheld the finding, my lab manager, bench mate, primary investigator, or some other nosey colleague would find out about it or another genius from another lab halfway around the world would figure it out (the theory of evolution was created by Darwin and another man, Wallace, essentially simultaneously and on opposite sides of England) and publish it themselves, robbing me of my opportunity for fame and fortune. The intellectual property that a cure for AIDS represents is a goldmine. Think, tenure at Harvard or head of the WHO (Francis Collins’ involvement with the human genome project earned him the position as head of the NIH). Hopefully I’ve illustrated why it’s virtually impossible and downright silly to cover up an important scientific discovery.
This brings me to my next favorite conspiracy: scientists are excluding creationists from contributing to scientific literature. Again, the logic is there: creationism (or intelligent design) and evolution compete for the most plausible theory of the origin of life on earth so it makes sense that the group in power exclude their rival from the competition and silence them from the literature.
Still, the rationale is flawed. Proving creationism means proving the existence of God. Do you have any idea how important you would become if you proved the existence of God? Anyone who did that would be as well known and as powerful as the Pope or Mother Teresa. Stop me when my explanation of why this conspiracy theory is bunk sounds familiar. Maybe I’ll just stop myself.
Hopefully I’ve managed to illustrate why my pet conspiracy theories are little more than that. Any questions?