People only get tested when they want to know if they're sick or are likely to have gotten sick. Media plays positivity rate of WuFlu tests like it's an indicator of actual spread, what if we correlated every test returning bad blood pressure as an indicator of a blood pressure disease epidemic? This does not take into account that normal people would hardly get tested for it. And of course people would get tested for bad blood pressure at higher rates if they knew something was wrong than those who don't care to get tested at all because they know they're healthy.