请各位指教：答案怎么是C？

since 1975 there has been in the united states a dramatic decline in the incidence of traditional childhood diseases such as measles. This decline has been accompanied by an increased incidence of Peterson's disease, a hitherto rare viral infection, among children. Few adults, however, have been affected by the disease.

which of the following , if true, would best help to explain the increased incidence of Peterson's disease among children?

a, Hereditary factors determine in part the degree to which a person in susceptible to the virus that causes Peterson's disease.

b.the decrease in traditional childhood idseases and the accompanying increase in Peterson's disease have not been found in any other country.

c.Children who contract measles develop an immunity to the virus that causes Peterson's disease.

d.Persons who did not contract measles in child-hood might contract measles in adulthood, in which case the consequences of the disease would generally be more severe.

e.those who have contracted Peterson's disease are at increased risk of contracting chicken pox.

correct choice : c

c怎么对呢？得过measles的children develop an immunity to the virus,不就说明他们不会得Peterson's disease了？那怎么能解释Peterson's disease 上升的现象呢？