I've been running into people lately who believe we haven't been to the Moon or even space. In the last two cases, these were college educated people working in an engineering/technology field. One guy claims that radiation would kill anyone trying to go to the Moon. And that the U.S. has only achieved low earth orbit (though we have sent robots to the moon).
The second guy was teaching a seminar on how to use an x-ray fluorescence instrument. He said we hadn't been to the Moon too. I would have questioned him more but I needed to know how to use the instrument. He also claimed to be a "flat earther". I simply replied with "Oh".
Then there are people who say we haven't been back to the Moon because of aliens. One nutjob astronaut (Apollo 17) has been claiming that aliens have been stopping nuclear war on Earth, and I believe he is contributing to that conspiracy.
So is it mental illness, ignorance, a combination of the two, or something entirely different? I read once that people who believe in conspiracy theories are low on the neurotransmitter serotonin.
Ironically, two of the people who believe aliens have prevented us from going back to the Moon also believe that 9-11 was an inside job. They are also veterans (one a M1 tank commander in Desert Storm, another a Chief Petty Officer in the Navy).