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A positive test result does not imply a high probability of illness because the base rate (prevalence) of the disease is very low compared to the healthy population. Even with high sensitivity and specificity, the sheer number of healthy individuals produces a large absolute number of false positives.

Conditions: The disease prevalence is $1\%$.; The test has $95\%$ sensitivity and $95\%$ specificity.; The population is a hypothetical model of 2,000 people.

A positive test result does not imply a high probability of illness because the base rate (prevalence) of the disease is very low compared to the healthy population. Even with high sensitivity and specificity, the sheer number of healthy individuals produces a large absolute number of false positives.

Conditions: The disease prevalence is $1\%$.; The test has $95\%$ sensitivity and $95\%$ specificity.; The population is a hypothetical model of 2,000 people.