Slapped cheek syndrome is caused by a virus called parvovirus B19. Also known as "erythema infectiosum" or "fifth disease", it is a common childhood disease, and has become more common recently. It is a mild disease in children, causing a slapped cheek appearance. Interestingly, the rash occurs after the child stops being contagious (unless they also have a rare complication): a child with a rash is not contagious, but they may have been incubating the condition for the previous 4 to 20 days. As the illness is usually only diagnosed after the rash has appeared, there is seldom any reason to exclude children with the condition from school.
About 60% of us contract the condition as children, and this gives us life-long immunity to future episodes. People with immunity cannot catch the virus again.
Rarely, if a woman catches the virus while pregnant, the baby may also catch the virus. If a woman becomes infected before the first 20 weeks of pregnancy there is an increased risk of miscarriage of about 10%. Nothing can be done to prevent miscarriages due to parvovirus B19 infection.
Rarely parvovirus B19 infection may make the baby anaemic, which can cause a condition known as hydrops fetalis. Women who become infected between 9 and 20 weeks of pregnancy have a 3% risk of hydrops fetalis. This can cause a stillbirth. It can, however, be diagnosed from ultrasound scans, and treatment may prevent a stillbirth.
As most adult women are already immune to parvovirus B19, fewer than 1% of women who are exposed to the virus between 9 and 20 weeks of pregnancy will develop hydrops fetalis. (See appendix for details of this calculation.)
TAKEN FROM www.fam-english.demon.co.uk/parvovir.htm