One of the common tests for the Lyme disease has a very high false positive rate. My friend got scammed by a pseudo-doctor abusing this fact.

Basically his father had huge health problems and they ran out of options so they went to that guy. It was obvious bullshit (diagnosing "parasites" with "electric resonanse", such stupid stuff, but they did it cause why not, it's not like it's going to make it worse). The guy also "tested" my friend and told him he has Lyme and should take this test in a normal medical clinic. My friend did and the result was positive. So he believed the guy for a while and took his "medicines" until he found out that test has 57% false positive rate :)

After he took the other test and repeated it several times it turned out he doesn't have Lyme.

Possibly that means "test positive" like when the CDC says "1/3rd of all Americans test positive for spinal meningitis" a surprising but true statistic. But spinal meningitis is asymptomatic in the overwhelming majority of people who test positive.

That presumably needs to make some assumption about it being non-symptomatic and endemic in way that most people can't test for. Which makes it essentially a matter of faith.