Say 1 in 10 people have flu and 1 in 10 have covid. In a population of 100, 9 have just flu, 9 have just covid, and 1 has both. Those 19 people present similar

Say 1 in 10 people have flu and 1 in 10 have covid. In a population of 100, 9 have just flu, 9 have just covid, and 1 has both. Those 19 people present similar symptoms, and anyone among them has roughly a 53% chance of having each disease. They take a flu test and it comes up positive. Now they're either one of the 9 with just flu, or the one unlucky person with both, so there's only a 10% chance they have covid. The impact of this effect changes if has-flu and has-covid are correlated, but it's still there unless it's a 100% correlation.