Explain why some people are naturally resistant to infection by the virus that causes fifth disease

Fifth disease is a mild illness caused by a virus called human parvovirus B19. The medical name for fifth disease is erythema infectiosum (EI). This infection occurs primarily during winter and spring, most commonly in children between 5-14 years of age. Initially, fifth disease causes an erythematous (reddish) rash on the child's face appearing as though the child had been slapped on both cheeks. Sometimes in North America, the disease has been referred to as "slapped cheek syndrome" or simply "slapcheek." The characteristic appearance of the rash gave rise to the names "apple sickness" (or ringo-byou) in Japan and "butterfly pox" in Hungary (since the cheeks resemble the wings of a butterfly).

This post might have the info you need: