One of the reasons the results seem counterintuitive is that the "a priori" probability of someone who comes to the clinic is generally much higher than the prevalence of the disease in the general population. About 20% of the general population has diseasitis. Of the population that comes to the clinic (generally because they have symptoms or has been in close contact with someone with diseasitis) that percentage is likely much higher.
One of the reasons the results seem counterintuitive is that the "a priori" probability of someone who comes to the clinic is generally much higher than the prevalence of the disease in the general population. About 20% of the general population has diseasitis. Of the population that comes to the clinic (generally because they have symptoms or has been in close contact with someone with diseasitis) that percentage is likely much higher.