tl;dr - I didn't think this post would be that long.
This post about rare diseases is based on an article I read back in med school. Written by one of my teachers in pathology, Ole Didrik Lærum. He was quite funny, and said article was mostly meant to be funny.
Rare diseases can be grouped in 4 categories.
1. Diseases that appear to be rare because they're not diagnosed.
These are conditions with no or very vague symptoms. Until we see symptoms of organ damage (may be caused by many pathological conditions) which may take years. Among common diseases that are easily overlooked we find arterial hypertension, type 2 diabetes and glaucoma (increase pressure inside the eye). In most cases the only way to detect them early is by measuring blood pressure, blood glucose and eye-pressure, and largely that depends on the affected going to a GP on their own initiative. A piece of free advice: when you pass the age of 40, you should do exactly that.
Another example is phenylketonuria, a congenital lack of enzymes handling the amino acid phenylalanine. Untreated this leadds to among other things brain damage. Eventually. It occurs in 1 of 12000 new born, which means that in Norway there are 4-5 children born with it each year. In Norway we routinely take a blood sample from newborns to detect the condition, otherwise we would catch it before it was too late. I assume this is not possible in many 3rd world countries.
A third example: There are diseases where symptoms vary depending on gender. Myocardial infraction in women often differ from the typical symptoms in men, and since heart disease for a long time was much more common in men, those are the symptoms we're usually looking for. So we tend to miss more infractions in women than men. In addition for 40-50 year old women, we may incorrectly attribute symptoms to the menopause.
2. Diseases that are rare because they are rare.
Currently we've classified approximately 7000 rare diseases, some of them affecting only a handful of patients. One example is Dylan, suffering from
Partial Triplication 15 - a mutation of his 15th chromosome (
https://www.bbc.com/news/disability-35640807). Only 19 cases are known globally (2016), it's so rare that it isn't even mentioned in Wikipedia. Some diseases are even rarer, with only 1-2 known cases.
BTW, Phenylketonuria can be treated by avoiding food rich in phenylalanine. You may have noticed that certain foods a marked with a warning, saying it is a source of said amino acid.
Most of these conditions affect children, and in the majority of cases there is no cure. Also, with that many diseases it often takes time to establish the diagnosis.
3. Diseases that hasn't happened. Yet.
For instance depending on where you are. One example is muscular and skeleton atrophy caused by long periods of weightlessness. Before we went to space and stayed there for months this was not an issue. If we establish permanent residence on the moon, I would not be surprised if we discovered diseases caused by for instance the regolith. And if we go to Mars, even more bad things may reveal themselves.
4. Diseases that haven't been detected, and maybe never will (the joke part).
We may still describe them, like the renal podocytoma (Ole Didrik Lærum). Podocytes are special cells found in the kidney.
A podocytoma would be a benign tumor of said cell type (if it was malign we would call it a podocytocarcinoma). At the time of writing (Lærum) this hasn't been found. We can still know something about it. One symptom would no doubt be hematuria (blood in the urine).
Don't think it's there now, but for some time it was listed in Guinness Book of Records (at least the Norwegian edition) as the rarest disease. Which makes sense, you can't find a disease rarer than 0 cases per year.
pibbuR who (sadly?) suffers only from very common diseases.
