I know it is terribly confusing...

Functional means that your ability to function is impaired.
It is a term used when TESTING does not show organic
disease. Blood work, MRI, Xrays, etc will reveal things, and then the functional term is not needed.

It is like with peripheral neuropathy... over 100 causes of this!
But many doctors can't or won't spend the effort to find the particular one for each patient. Instead they use Idiopathic, as a diagnosis, which means unknown. Some sensory neuropathies and the pain they cause totally elude testing, and hence doctors
are not interested for the most part with patients like this.
They just give palliative treatment, meaning symptom reduction.

I will suggest to you wild_cat, that dancers are well known for obsessive weight control. Three of them lived next to me at one time. What happens is that calorie restriction often includes lower fat intake, and the good fats, omega-3s get avoided.
These essential nutrients, form the blood brain barrier (BBB) of the brain and when that breaks down, nasty things in the blood can leak into the brain thru the microscopic openings that appear.

The BBB is often looked at for Parkinson's patients. There are many papers and some appear here on our PD forum discussing leakage of toxins, bacteria and viruses into the brain that the BBB would normally exclude. Your viral illness may have traveled this route and is inflaming your brain now. People really need to rest and have adequate nutrition when ill, because once these protective barriers are breached, fixing it is not going to be likely, when the brain is concerned. The most common sequel I've seen personally to viral patients who overdo...is cardiac emergencies.
The sac around the heart can be attacked by viruses and swell up compress the heart--- there is a fancy name for this-- cardiac tamponade from fluid building up in there--- which can be deadly. My son's physical education teacher had this from training when he was ill. It was a close call for him.

You can find many many papers posted on our PD forum about blood brain barrier research. Just use the search function at the top of every 1st page of each forum.

There are many things that doctors just can't grasp yet. Either it is too complex or all the data is not in yet. But with your viral,
link in there, it seems that it should be considered.