A school had tried very hard to uphold the requirement for all children to be vaccinated. However, state law allowed parents to exempt their child if they had a religious reason for refusal. Two children have been diagnosed with chickenpox. Which will be the most likely result?

Correct Answer: D

Rationale: Herd immunity is a state in which those not immune to an infectious agent will be protected if a certain proportion (generally considered to be $80 \%$ ) of the population has been vaccinated or is otherwise immune. Because most children in the school are immune, it will be difficult for the chickenpox to spread.

Correct Answer: B

Rationale: The positive predictive value is affected by what proportion of the tested population has the problem. To increase the positive predictive value, screen populations most at risk for the problem. Testing a large population at once, using a very sensitive or very specific screening test would not increase the positive predictive value of a screening test.

Correct Answer: C

Rationale: The problem with very sensitive tests is that the test picks up almost all people with the disease but also many others who do not have the disease. These 'false positives' result in persons needing follow-up diagnostic tests. Additional time, effort, and expense, as well as worry, result until the negative test results are obtained.

Correct Answer: B

Rationale: Incidence (B) measures new cases over a period, reflecting disease onset. Prevalence (A) is total cases, morbidity (C) is disease burden broadly, and mortality (D) is deaths, not new cases.

Correct Answer: C

Rationale: Case-control studies (C) compare diseased (cases) and non-diseased (controls) to assess past exposures. A measures prevalence, B follows exposure, D tests interventions.