A germaphobe is a person that fears (phobia is Greek for "fear") all sorts of germs. Another term used is mysophobia. Here's the link to the wikipedia page: [].
Sheldon is frequently seen showing typical signs of a germaphobe. When Penny returns from a family visit mentioning that "everyone" had got sick with the flu, he immediately asks her to leave and sprays the apartment with disinfectant (0111). He is constantly paranoid that somebody could touch his food and refuses to eat things that other people have touched (0202, 0207, 0223). He also believes that paper towels are the only sanitary way to dry one's hands because hot air blowers are "incubators and spewers of bacteria and pestilences" (0204).
To understand Sheldon's germophobia it should be noted that he was frequently sick as a child (0108). Being the scientific analytical mind he is it therefore only logical to avoid contact with germs at all costs.