|Does that mean if the bar is going to the right of the middle line, that the disease is increasing and if it stays to the left of the midline then it's decreasing?
That's correct. An increase or decrease on the weekly chart tells you what it's doing this month (the past 4 weeks) compared to the average over the last 5 years.
But you have to know what the rate of misdiagnosis has been over the last 5 years to know if what you're seeing is an actual real increase or decrease, or if it's just the "new thing" to test for that disease and diagnose it.
For example, for a couple of years that chart looked like pertussis was making some kind of big comeback...that chart made it look like it had suddenly surged through the roof. But really, the CDC just told doctors to start testing for pertussis, where in years past they assumed pertussis was virtually eradicated, and would just diagnose people with pertussis as having "asthma" or "atypical bronchitis".
The same thing happened with mumps.
So you have to cross reference a lot of stuff and put it all in perspective to see if what's being reported is reliable and real.