My understanding of this disease is they are exposed to the virus. Is there any consistant factors from each time she got it...a pet a friend ect..?
Are you sure this is slapped cheek syndrome that she is suffering from?
According to the NHS website slapped cheek syndrome is caused by a virus called parvovirus B19. Parvovirus B19 is an airborne virus that is spread in much the same way as the cold or flu viruses. It can be spread through coughs and sneezes that release tiny droplets of contaminated saliva which are then breathed in by another person.
It's very difficult to prevent the spread of the virus as people are most contagious before their symptoms begin, so they are unaware that they are infected.
Once you've been infected you should develop a lifelong immunity and not experience any further symptoms.
Because your daughter has had this problem when she was a child, unless she has a low immune system, technically she would have built up immunity to the virus.
As she is 19 and only her cheeks get red and hot, but she is not ill, this may be just normal flushing of the face due to embarrasment or a condition called Rosacea. Keep a record when she gets the red cheeks to see if they come around her menstrual cycle (sorry, but I do not know about special needs young females, but I presume they still do have menstrual cycles). I would appreciate you letting me know.
I would advise you to take her to the doctor when she has this, so that he can diagnose what this is so that it can be treated accordingly. If it is Rosacea, the doctor can prescribe some creams for this. The one I use is called Rosex and it helps me.
When you take her out in the sunshine make sure she has a sunblock on, and a large brimmed hat to protect her skin. You may find that she gets the red cheeks when she is hot or when she has cold wind blowing on her. Keeping a diary when she gets the red cheeks will be useful to see a pattern when this happens.
Make sure she has plenty of fluids to drink.
Best wishes