I frequently confuse the terms iatrogenic and idiopathic with one another, perhaps because both begin with the letter “I” and end in “c.”  They, of course,  pertain to medicine but have completely different meanings.  

An iatrogenic disease or symptom is one induced in a patient by the treatment, examination or comments of a physician. (Iatros is Greek for doctor, genic relates to causing.)  Hospital-acquired infections, anesthesia mishaps, falls and drug errors are the most common iatrogenic events. Iatrogenic disease is a major cause of morbidity, prolongation of hospitalization and even death.  

Idiopathic, on the other hand, relates to conditions whose symptoms can be described but have no known cause. (From  Greek, idios "one's own" and  pathos "suffering," idiopathy means approximately "a disease of its own kind".)  

According to Jennifer Kahn, whose column was published in a recent New York Times magazine,  “Before germs were understood, most diseases were idiopathic by definition, including the Black Death, which we now know was caused by a bacterium (Yersinia pestis) but which doctors at the time hypothesized might be caused by staring at someone that was ill, the alignment of the planets, bad smells or wearing pointed shoes.  

“What is startling is how many mystery diseases still exist today. More than one-third of acute respiratory diseases are idiopathic; the same is true for up to 40% of gastrointestinal disorders and more than half the cases of encephalitis (swelling of the brain).  Up to 20% of cancers and a substantial portion of autoimmune diseases, including multiple sclerosis and rheumatoid arthritis, are thought to have viral triggers, but a vast majority of those have yet to be identified.”

Strangely, the sixth edition of “Diseases of the Human body,” has very little to say about idiopathic diseases, only that some diseases, having no known cause are described as being idiopathic and must be treated symptomatically.  

In terms of iatrogenic diseases, however, the book notes that they are caused  by medical treatment and its effects. For example, the chemotherapy drugs used to treat some cancers can  cause severe anemia, or hepatitis can develop as a result  of a contaminated blood transfusion.

The wise thing to do is to watch for rare cases of mystery illnesses in people, which often exist before the organism gains traction and is able to spread. One example based on retrospective blood samples, is that scientists now know that HIV emerged nearly a dozen times over a century starting in the 1920s, before it went global.  Zika was a relatively harmless illness before a single mutation, in 2013, gave the virus the ability to enter and damage brain cells.  

In the past scientists identified organisms in a sample by culturing them.  The organism is grown on a petri dish and examined under a microscope to understand just what it is.  But because less than 2% of bacteria and even fewer viruses can be grown in a lab, this process often reveals only a tiny fraction of what is actually there.  A newer method known as metagenomic sequencing may be the answer.  This technique can be applied to a messy sample of just about anything including blood and nasal drippings, which often contain dozens or hundreds of different organisms, all unknown, and each with its own DNA.  

In order to read all of the fragmented genetic material, metagenomic sequencing uses sophisticated software to stitch the pieces together by matching overlapping segments. The assembled genomes are then compared against a vast database of all known genetic sequences maintained by the government’s National Center for Biotechnology Information which makes it possible to identify everything in the mix.

Metagenomic sequencing can also be used to sample the environment, identifying every type of bacteria present in the human gut, or in a teaspoonful of seawater.  These studies have found more than 1,000 different kinds of viruses in a tiny amount of human stool, and another found a million in a few pounds of marine sediment.  Most were organisms never seen before.

There have also been historical mysterious diseases that appear suddenly and then strangely disappear.  According to Wikipedia, they are defined as diseases for which the cause has not yet been identified. Reasons are lack of identification of etiology (cause) include lack of professional interest, difficult access, and lack of resources, in addition to being unknown to medicine.

There are a number of examples, one is "Sweating Disease."  It first reared its ugly head in England in the summer 1485 and there were four further outbreaks - in 1508, 1517, 1528 and 1551 - before completely disappearing and  never to be seen in that land again.  It was highly contagious and decimated settlements around England taking thousands of lives. In fact, towns found themselves fortunate if half the population survived.  Although studies have suggested that it was not as lethal as the plague, sweating illness caused shock and horror because of the sudden onset and lethality.  

Max Sherman is a medical writer and pharmacist retired from the medical device industry.  His new book “Science Snippets” is available from Amazon and other book sellers. It contains a number of previously published columns.  He can be reached by email at  maxsherman339@gmail.com.