You're right to be concerned about hygiene but make sure you are reasonable in your request.
The best way to initiate this process would be to just call the teacher and express your concern...about the flu, not the "you suck as a teacher because you won't force kids to wash their hands!".
Your conversation could go something like this: "In light of the recent problem with swine flu, I'm left feeling a little concerned that my son or maybe even other children might not know the importance of frequent hand-washing and taking general precautions. I just want to make sure my son is able to and capable of making good choices when it comes to his personal health. How can you and I work as a team to help him with this? What kinds of things have you been doing with the class/requiring the kids to do as far as hand-washing is concerned?"
This approach makes it come from what you and your son can do to help himself, not as an accusatory overreaction to what she may or may not be doing. In using an approach like this, I can guarantee you'll get a much better response than just threatening to go to the principal if your demands aren't met immediately.