Good question, and one that can have several outcomes with its answers.
1) If you're an employer, order them to wash their hands. Failure to comply is a direct disobeyment of the Health and Safety Act of 1979.
2) If you're in employee, remind them that it could cost them their job, and also a customer/several customer's good health.
3) If you're a customer, question why they didn't wash their hands and if they don't answer/don't give a valid reason, make a complaint to the manager and leave.