Dividing the absolute frequencies in the 2x2 table by the total number of events N = A + B + C + D, we obtain a useful model (sens=sensitivity, spec=specificity):

  Disease present (D+) Disease absent (D-)
Test result positive (T+) P(D) x sens [1 - P(D)] x (1 - spec)
Test result negative (T-) P(D) x (1 - sens) [1 - P(D)] x spec
  A + B (diseased subjects) C + D (healthy subjects)

Now we are ready to derive a famous formula for the conditional probability P(D+/T+)

This is the famous

Bayes' formula.