Fifth's disease is the fifth of the common childhood rash diseases, the others being Measles, Rubella, Chicken Pox and Roseola.
My daughter had this when she was small, and it did, indeed, look like someone slapped her in the face. In fact, I assumed it was her older brother (who probably had good reason for it as she was going thru a particularly pesky stage).
Fifth's disease is not serious for most children or adults, but it is somewhat akin to German measles in that it can be a hazard to pregnant women and their unborn babies. If you are pregnant and have it or have been exposed, you should definitely see your doctor.
BethMI