Final Answer:
In the doctor's "Day 1001" journal entry, the main difference between the doctor and Sam's immune systems is that B) Sam is immune to the virus, while the doctor is not.
Step-by-step explanation:
The scenario presented in the question indicates that Sam has developed immunity to the virus, while the doctor has not. This could be due to various factors, such as Sam being exposed to the virus and successfully mounting an immune response, possibly leading to the production of antibodies or other immune factors that confer protection.
On the other hand, the doctor may not have encountered the virus or might not have developed immunity for some other reason. Option B correctly captures the distinction in their immune status.
Option B is the answer.