Fifth disease, also called erythema infectiosum, is caused by infection with parvovirus B19.  It is common in children and usually mild.  The most recognized feature fifth disease is the “slapped cheek” rash.  However, parvovirus B19 infection can be serious in people with weakened immune systems.  Fifth disease may also be a concern for women who are not immune (i.e., not had the disease previously) who are infected during pregnancy, but usually their babies do not have any problems (see https://www.cdc.gov/parvovirusb19/pregnancy.html).