Answer:
The correct answer is - the ancient observation that someone who had recovered from the plague could safely care for those newly diseased.
Step-by-step explanation:
Immunological memory is the ability of the body to recognize or identify and kill or remove the antigen, very quickly and faster, the body has previously encountered and developed an immunological response.
These memories are developed due to memory B cells or, memory CD8+ T cells and memory CD4+ cells developed by the first-time immune response. If someone who had recovered from the plague must have developed memory cells for the antigen and safer than those newly diseased.