I don't believe there is a disease called fibromyalgia, it doesn't sound real.
That's not good. Have you ever met anyone or known anyone who had fibromyalgia?
Maybe it exists but I don't believe there is one.
It may sound a little disrespectful to claim that a disease does not exist. This may be a little off putting for anyone who has the disease. Also, there is the internet that you can use to lookup the disease to find out whether it is real or not anyway.
It does exist, but as a myth. It is just a story and that's all there is to it.
But it is not and thinking that it is not real is disrespecting people who are suffering from that disease. It could also cause you to ignore any symptoms or signs that somebody you know is suffering from the illness.