This is exactly how my OCD often plays out. I spend a great deal of time trying to anticipate and avoid situations that might trigger strong obsessive-compulsive cycles. When I return to awareness and realize that I've failed to keep watch, I become terrified that something awful has happened in the interim.
So you laugh. It's funny. Because it's a joke anyway, right? People like you are funny, comic relief. People to make fun of and to brush off. Silly germophobia. Silly handwashing and rituals and constant unwanted thoughts. You laugh because it's easier than facing the fact that you've lived your whole life with this.
Just as Hannah herself resists typical far-fetched sitcom stereotypes -- she's not model thin, she struggles with her finances and her career choices-- her OCD symptoms are presented in a way that resists the low-hanging fruit of a kooky character most of us never encounter in our day-to-day routine.