n. A person who is obsessed with cleanliness, particularly the avoidance and elimination of germs and bacteria.
Other Forms
Favorite symbols of the movement (and of its detractors) are the obsessive-compulsive television detective Monk; the famously germ-averse game show host Howie Mandel, who has a sign on his Web site showing a slash through a handshake; and the mysophobe (the proper term for what most people call a germaphobe) Howard Hughes.
—Allen Salkin, “Germs Never Sleep,” The New York Times, November 05, 2006
Bob Saget hosts "1 vs. 100," following in the footsteps of "Deal's" Howie Mandel, whom he gently mocks by refusing to shake hands with the first player (Mandel is a germaphobe).
—Rob Owen, “Network hopes to strike another 'Deal' with new show,” Pittsburgh Post-Gazette, October 13, 2006
1995 (earliest)
The Howard Hughes Medical Institute, incidentally, was named after a lifelong germaphobe who became a billionaire by selling TWA at the top. Rockefeller University was named after a man who became a billionaire by taking in dollars and giving away dimes.
—Alan Abelson, “Up And Down Wall Street,” Barron's, July 29, 1995
This is one of those words that seems like it's been around forever, so I was surprised to find not only that it doesn't appear in any mainstream dictionaries, but that it only bobbed to the surface of the language around 1995.

The New York Times citation mentions that the "proper" term for the germ-hater is mysophobe, which comes from mysophobia (1879), a condition that the Oxford English Dictionary defines as the "irrational fear of dirt or defilement." Mysophobe may in some sense be more proper, but it's unlikely to become the term of choice since germaphobe is such a straightforward and easily grasped construction. And, indeed, germaphobe outsearches mysophobe by more than 3-to-1 on Nexis and by more than 2-to-1 on Google.