&gt; Something is not clear to me. How do you know that a false positive is, in fact, a false positive?<p>Bob has a full body MRI. THat returns some shadowy spots on his lungs. He&#x27;s asked if he used to smoke (he did), and if he had any childhood illnesses (german measles, measles, chicken pox, etc) (he did).<p>Are those spots early cancer? Or are they just scarring from the childhood illness?<p>One way is to cut Bob open and have a look. Or jab him with a massive biopsy needle. Except, now you know that this one spot isn&#x27;t cancer, it was a scar.  How about all the rest?  That one biopsy increased Bob&#x27;s risk of harm.<p>&gt; What&#x27;s the argument for not testing in the first place?<p>Testing often doesn&#x27;t give any useful information. It often gives wrong information - people who have a disease are told they don&#x27;t have it, and people who don&#x27;t have it are told they do have it.  Many of these people then go on to have treatment.  All treatment carries some risk of harm.<p>You need to know if the benefits of testing (especially of testing a large, mostly healthy population) outweigh the risks of providing risky treatment to those people, some of who will not have the disease.<p>You also have to see what happens if you just don&#x27;t do anything.  Prostate cancer is a good example here.  Some people will die of it - they have an aggressive cancer that will kill them rapidly.  But many people will die with it - they have a slow growing tumour, and they&#x27;ll die in old age from something else.  Treatment for prostate cancer is pretty harsh.