Have you ever taken a class in school and right after the test everything you learned is gone? Well, doctor's are no different. They don't know or even remember everything but it's not right for a doctor to call you crazy and if you have the option. I would suggest another doctor. As far as a parasite being invisible to the naked eye, I say rubbish!!!!
According to Wikipedia, The adult demodex mites are only 0.3Ė0.4 millimeter. That's a little under 1/2 a millimeter. If you look at your ruler, you can see what 1/2 a millimeter looks like. It's most definitely visible.