"Trump maintained that Obama's birthplace is a matter of opinion, rather than fact." This is why birthers, creationists and anti-immunization wackos can't wrap their heads around the fact that they're just plain wrong. GET OVER IT.
Also read an interesting article about why otherwise intelligent medical professionals and scientists sometimes go off the rails and buy into crank science and conspiracy theories (like Luc Montagnier, co-discoverer of the link between the HIV virus and AIDS, who's thrown his hat in the ring with Jenny McCarthy and the "vaccinations cause autism" idiots):
"How do scientists become cranks and doctors quacks?"