Based on what you know about disease ecology, what can you...

1. Based on what you know about disease ecology, what can you speculate about the herd immunity threshold necessary to control a disease marked by "D" in the figure? Briefly, why?
2. Which letter on the graph represents a region in which you would expect the fewest number of diseases, and why?
3. Antibody tests (or serological tests) are useful to detect past exposure to a particular virus, but they may also incorrectly indicate past exposure even when there has been none. What is this phenomenon called and briefly explain why this issue is important for small samples (that is, when few people are sampled)?