A fairly common childhood complaint; it is estimated that 60% of us contract it during childhood.
It is caused by parvovirus B19 and is also known as erythema infectiosum and the fifth disease. liveforever says that, in Denmark, it's called 'lussingesyge', meaning '(cheek)slap-disease'. Contracting it results in lifelong immunity. The Public Health Laboratory Service say:
"Parvovirus B19 is a single-stranded DNA virus belonging to the Parvoviridae family of viruses, which includes a number of animal parvoviruses such as the canine parvovirus and feline panleukopenia virus. Parvoviruses are species specific and B19 is the only known pathogenic human parvovirus. The virus is known to replicate in rapidly dividing erythroid progenitor cells. Other target cells are less well defined and may include myocardial tissue."1
Its symptoms are a facial rash (hence the name) and joint pain and/or swelling (children usually just get the rash). During the course of the disease, the rash may spread over the arms and torso. If caught in adulthood, it is more severe, with the potential to cause joint problems for several months. Another complication is that, if a woman catches it during the first 20 weeks of pregnancy, the foetus could become anaemic or a miscarriage could occur. However, most pregnant women will not contract it and, of those who do, most will not experience serious problems. Obviously, people with a suppressed immune system (HIV-positive people, transplant patients etc.) need to be more worried than healthy people, but then that applies to any infectious disease.
The virus is airborne (though, more rarely, you can catch it from contact with the blood or urine from an infected person) and the illness is contagious for about 4-20 days between exposure and the rash appearing. Once the rash appears, the patient is no longer contagious, meaning that there is little point taking the person out of school (unless they feel ill, obviously). As the person is contagious before any symptoms show up, only blood tests will show if a person is carrying the virus.
There is no vaccine available, and no treatment for the disease itself is necessary, though symptoms such as fever and joint pain may need treatment. Prevention is difficult for the reasons above, though basic hygene is always a good idea for preventing the spread of disease.
The Public Health Laboratory Service Facts and Figures Website; entry on 'Slapped Cheek Syndrome'; http://www.phls.co.uk/facts/parvovirus.htm