Fifth disease (so called because it is the "5th disease" on the
standard list of ones that children typically get) is caused by a
virus. As such, once you've had the disease (and about half of
Americans get it by the time they're adults) you can't get it again.
This is true of all viruses - and is why vaccines work (though there's
no vaccine against Fifth).
People who are anemic, or "have leukemia or cancer, who are born with
immune deficiencies, who have received an organ transplant, or who
have human immunodeficiency virus (HIV) infection are at risk for
serious illness due to parvovirus B19 infection." (quotation from the
CDC page listed below) Adults with these conditions will probably
have a harder time fighting off the infection.
Here are some good resources for information on Fifth Disease:
CDC: Parvovirus B19 (Fifth Disease)
KidsHealth: Fifth Disease
The information on this site is more appropriate for children with the
disease, but there's some applicable stuff for adults too.
Mayo Clinic: Parvovirus Infection
I got these links from the MedlinePlus health topic for Fifth Disease
Please let me know if you need more information!