The interviewer needs to hear that you are a team player and do not allow situations like this to get the best of you. Tell the interviewer that you recognize your co-workers may get sick from time-to-time, and it is your responsibility as a member of the team to step up during those times to help you. Share that you would simply keep a positive attitude and cheerful customer service attitude serving each customer as quickly as you can. Be sure to mention that if there is another cashier who might be able to come in that day to help you, you may consider reaching out to them to ask if they would be willing to come in for a few hours as well.