The Republican front-runner has called himself a "clean hands freak" and has referred to hand-shaking as "one of the curses of American society," according to The Washington Post. In fact, Trump doesn't even like to push a ground floor elevator button, according to Mother Jones, because it has been touched by so many others.
But what really is germaphobia, scientifically speaking -- and is Trump right to be grossed out by other people's hands?
Germaphobia, also known by the medical term mysophobia, is defined by high levels of distress when a person is confronted with real or imagined dirtiness and contamination.
At its most severe, it's a form of obsessive-compulsive disorder, a mental health condition characterized by obsessive thoughts (for instance, worries that a surface is dirty or a fear of harm coming to loved ones) and compulsive actions performed as a way to rid oneself of the thoughts.
On a milder, subclinical level, however, many people have some level of concern over germ exposure -- and those worries aren't completely off-base.
When it comes to shaking hands in particular, there is some validity to Trump's concerns. Our hands can carry all sorts of germs, including bacteria and viruses, Dr. David Whitworth, a biochemist at Aberystwyth University who has studied the transmission of infections, told The Huffington Post in an email.
"Flu can be transferred by touch, but so can fecal bacteria and hospital superbugs like MRSA" or Methicillin-resistant Staphylococcus aureus, he said.
Infections like the norovirus spread easily by hand-to-hand contact, and one study conducted by Whitworth and his colleagues found that a single handshake can transfer hundreds of millions of colony-forming units of E. Coli -- that is, after volunteers in the study had been exposed to the bacteria.
So, instead of shaking hands, the 2014 research, published in the American Journal of Infection Control, suggested some better alternatives for greetings. The findings revealed that a high-five transmits only half as many germs as many germs as a handshake, while a fist bump transmits only one-twentieth of the germs of a handshake.
"We found that handshakes can transfer many more bacteria than high-fives or fist bumps, so the nature of the contact matters," Whitworth said. "Contamination also depends on hand hygiene habits."
A big part of the problem is that many people don't have very good hand hygiene habits. A whopping 33 percent of Americans don't wash their hands after going to the bathroom and 70 percent only rinse with water, according to one estimate, thereby increasing the spread of germs.
Statistically speaking, you're unlikely to contract anything serious from a handshake in a non-hospital setting, Whitworth said. But you could easily contract a cold or flu, so best to wash your hands regularly (with soap!) and don't sneeze or cough into your palms -- for everyone's sake.