As discussed in yesterday's blog, it's not a numerical accident that rare diseases are rare. Biological processes account for the rarity of certain diseases, and for the commonality of common diseases. In my book Rare Diseases and Orphan Drugs: Keys to Understanding and Treating the Common Diseases, I explore the fundamental biological differences between rare diseases and common diseases. Here are six observations that distinguish common diseases from rare diseases
1. Rare diseases typically occur in a young population. Common diseases typically
occur in adults, increasing in frequency with age.
2. Rare diseases usually occur with a Mendelian pattern of inheritance. The
most common diseases may sometimes cluster in families, but they are,
without exception, non-Mendelian.
3. Rare diseases often occur as syndromes, involving several organs or physiologic
systems, often in surprising ways; most common diseases are non-syndromic. [A syndrome is a constellation of pathologic features associated
with a single disease or condition, usually involving multiple organs. For example, inherited
deafness is often syndromic. Syndromic deafness is accompanied by other abnormalities,
possibly involving facial structure or nerve function. Non-syndromic deafness
affects hearing and no other structures or functions.]
4. Environmental factors play a major role in the cause of common diseases;
much less so in the inherited rare diseases.
5. The difference in rates of occurrence of the rare diseases compared with the
common diseases is profound, often on the order of a thousand-fold, and
sometimes on the order of a million-fold.
6. There are many more rare diseases than there are common diseases.
I urge you to read more about this book. There's a good preview of the book at the Google Books site. If you like the book, please request your librarian to purchase a copy of this book for your library or reading room.
-Jules J. Berman, Ph.D., M.D.