Dave Hoover writes about a talk he gave on washing your hands:
This is something Uncle Bob started talking about 2 years ago: “It has become my position that TDD is a necessary discipline for professional developers. I consider it rather like sterile procedure for doctors. It's simply what you have to do to write professional code.” ... I find this analogy particularly appropriate when you have an existing codebase and you've been asked to fix or enhance something. Like a surgeon you need to ensure you do not infect your patient, and in order to prevent that, you need to create a sterile environment. If you don't have tests in place, you retrofit them in order to prevent regressions (similar to cleaning a dirty patient).
He goes on to reference the “First, do no harm” principle amongst other observations.