Incidence vs. Prevalence
The number that you see of 1 in 100,000 is the incidence rate, which is the number of people who are diagnosed with the disease each year. The prevalence is the number of people who have the disease at any one time. Prevalence does not mean the number of people who have ever had the disease. Otherwise, the prevalence of colds would be very nearly 1 in 1.
Dr. Arthur Asbury used the number of 3500 GBS cases in the US each year. That would be an incidence rate of 1.17 per 100,000. However, significant digits would require that we round that to 1.2 per 100,000.
For acute diseases, and in spite of the complications that can follow GBS, it is an acute disease, the incidence rate and the prevalence should be very nearly the same. For chronic diseases, the incidence rate and the prevalence should be very different, roughly the incidence rate times the expected lifetime of a person at the average age of onset. The prevelance rate for significant residuals from acute diseases, like GBS, would be roughly the incidence rate times the expected lifetime at average age of onset times the fraction with significant residuals.
A 95% confidence interval means that there is only a 5% chance that the mean of the population is outside the interval. We will have the wrong population if the disease is underdiagnosed. But if it is properly diagnosed, then the number has only a 5% chance of being outside the range. To get to 8 per 100,000, GBS would have to be misdiagnosed 87% of the time.