1. Whole groups of people were not infected by the disease. Only some people within each group were infected. 2. Whole groups of people were not infected by the disease. All the people within these groups seemed to be immune.
Therefore I would say it is ambiguous, though the second meaning is the first th
I interpret the statement as the following. Suppose there is a city through which a disease rages. Many people in the city are infected, however certain (whole) groups of people in the city are not affected, for example: no pregnant mothers have been infected; no one over the age of 70 has been infected; no one under the age of 3 has been infected.