Are you meaning "what she needs to worry about"...or "what she is more than likely worrying about" at the moment, that she doesn't need to? I am gonna take a stab at this one, and guess that you are talking about the latter - as a healthy young lady of 12 years of age, really doesn't need to worry about much of anything, unless she has health problems of course. She can not spend her days worrying about whether or not she is going to catch something, or be overly obsessive about germs...as this will eventually turn her into a hypochondriac. She "does" need to know to wash her hands often, as that is how germs are transmitted, to cover her mouth if she sneezes or coughs, not to share drinks, food, combs, brushes, hats, or shoes with anyone...just the common everyday stuff...my 12 year old plays with fishing worms, frogs, and baby turtles...of course she is a "country girl" and healthy as a horse. The things that she more than likely "is" or "will be" worrying about, is her first period. Like I said before, if she has had health class yet at school - as my daughter has, then she will have in the back of her mind about sexually transmitted diseases as she approaches that age, she could worry about something she has heard at school, on the radio, or TV, such as West Nile Virus and mosquito's, maybe spider or bug bites and bee stings, nothing serious...she should not be worrying about cancer, liver disease, kidney disease, copd, or heart problems...like I said, unless maybe she has overheard someone in the family talking about another family member, that has gotten ill or died from one of these diseases and she is afraid that she will get it also...it is just hard to say. Have you tried talking to her and asking her why she is worried about health problems, and what ones she is worried about? She should tell you. Like I said above, she will worry about little things like pimples, cold sores, getting the flu before her dance at school, but it should be nothing major. Children get their "cue's" from us. If "we" make a big deal of something as they are growing up, then they will also. Keep the lines of communication open with her, and let her know that it is OK to ask questions about anything, and that everyone gets ill occasionally. That is how we build our immune system and if she does get sick, she will get better. Without more specific information, I really don't know what else you are looking for, so I have given you a pretty wide view of things.
Good luck to you and your daughter.