Well about getting sick. My mom always said the reason her adult children don't get sick much is because we developed such a good immunity to everything from bringing home everything as children.
Being shy is an inherited trait I believe. I've read studies that say as much as 50% of your personality is inherited. Being shy is a personality trait I really can't relate to. Though I don't like to speak in front of large groups of adults, I have absolutely no fear of approaching individuals or small groups. My husband is from this area and I'm from down south. When we first met he would often ask me if I knew the people I was talking to in the grocery store or in line at the movies. He couldn't understand why I was talking to people I didn't know. I've broken the ice in his family (I'm white and they are all hispanic...no one knew how to react to the white women entering the family until I came along and broke down the barriers.)
At any rate, I have one daughter that would say hey to everyone that passed the grocery cart. I have one daughter that will absolutely start to cry if a group of people is looking at her. She will get hysterical if we walk into a room with a large group of people regardless if she knows them or not. I have talked to her and talked to her but she's just scared. She warms up eventually if there are kids around, but she won't interact with the adults much at all.
When my husband and I met I worked in a lounge. We had a big Halloween party with amazing prizes like a trip for two to the Bahamas. My husband's birthday is Halloween so he's always made a big deal out of dressing up. He looked amazing that night with real, movie quality makeup. Everyone commented all night how he would win the contest. When contest time came around he couldn't be found. Over an hour later he showed up. He told me he was so disturbed by the fact he would have to stand on the dance floor in front of the crowd that he went and hid in the car chain smoking for an hour until he knew the contest was over. He was a nervous wreck just thinking about it.
He's so much better after 16 years together, but he still gets super nervous in front of big groups. He would rather die than have everyone looking at him.
Our girls have never been around him in any kind of situation where he would exhibit this behavior, but the middle child definitely acts the same way. It had to be inherited.
So relax about it. It isn't the end of the world. There are 1000's of examples of famous, successful people that are/were shy. Just keep coaxing her and introducing her to smaller settings. Don't get upset with her. Keep working with her by introducing her to one child at a time. Show her in baby steps that there is nothing to be afraid of.