That means that your "99% accurate" test is wrong 9,999 times out of 10,000!
Funny, but this is not a correct statement. The test was also correct when it determined that 990,000 people were negative, and correct when it determined that 1 person was positive. It was incorrect 9,999 times out of 1 million: 99% accurate.
sounds_sound: Growing up, I hated that my parents decision to live out in the middle of nowhere was based our perceived safety. I always thought of it as a 'white people scared of the city' kind of attitude that was mostly unfounded. They never considered that driving 40 minutes to get anywhere was exponentially more dangerous. Now they live in Atlanta and literally drive the 50 meters to their neighbors house - clearly, they still don't get it :)
If you have an hour or so, do yourself a favor and listen to the Radiolab on Stochasticity: http://www.radiolab.org/2009/jun/15/ and if you tear through that, the one called Numbers will blow your mind.