Rare and Common Presentations and Conditions

Common presentations of rare conditions are more common than rare presentations of common conditions.

What does the above even mean?

There are a very large number — thousands — of individually rare conditions. So even though any single given rare condition is only rarely seen, having any rare condition whatsoever is relatively common. On the other hand, there are a limited number of truly common conditions: the common cold, reactive depression, and back pain, for example.

According to the geneticists, “A rare disease is generally considered to be a disease that affects fewer than 200,000 people in the United States at any given time. There are more than 6,800 rare diseases. Altogether, rare diseases affect an estimated 25 million to 30 million Americans.”

If we consider a rare presentation of a common disease to be defined similarly, this would mean that this would likewise be about a 1 in 1500 presentation of a common condition. If 1/3 of the US population has at least one common condition at any given time, this means that about 100,000,000 persons in the US have a common condition; if 1/1500 had a rare presentation, this would in turn mean that there are only about 70,000 persons with rare presentations of common conditions per year. This is much less than the over 25,000,000 with a rare condition.

So it turns out that, based on prevalence data, the number of people with a rare but otherwise typical condition significantly outnumbers the number of people with only a rare presentation of one of the common conditions.

The anxious patient doing an unfortunate Internet search for their diagnosis may err because they choose a rare presentation of a rare condition over the common presentation of a common condition from which they suffer, and not because they find a rare condition that truly fits them best.