Final answer:
The acute phase of a disease is when a person first becomes sick, while the convalescent phase is the period of recovery. A four-fold rise in antibody titer between the acute and convalescent phases indicates an immune response to the pathogen.
Step-by-step explanation:
The acute phase of a disease is the period during which a person first becomes sick and experiences symptoms. The convalescent phase, on the other hand, is the period of recovery after the acute phase, during which the person starts to feel better and symptoms gradually disappear.
A four-fold rise in antibody titer between the acute and convalescent phases is significant because it indicates an immune response to the pathogen. This increase in antibody titer suggests that the person's immune system is actively fighting the infection and producing more antibodies to neutralize the pathogen.