I am so sorry that you're nervous! You are going to be FINE! I've never had one myself though I have had quite a few family members and friends (including children) who have. They will certainly talk to you before the procedure. It's wise for you to tell them your concerns so that they can ease your fears.

You will be sedated which with relax your muscles and there won't be any gagging. They will be sure that you are comfortable and will ensure that you are out before the test is performed (it's really quick!). Remember that they do this multiple times a day and there won't be anything new in your case.

As far as viruses go, yes, they sometimes have outbreaks in the fall/winter in hospitals but #1, it's the end of the season, so I doubt that will be an issue and #2, the section of the hospital you'll be in certainly won't be contaminated with anything as they won't do procedures on ill people. The places that would be an 'issue' would be in the ER or admitted rooms, which you won't have any contact with.

You will be 100% FINE and will be so happy when it's over!