What becomes epidemic can often be assumed as normal.

A Cautionary Tale: Sudden Infant Death Syndrome And The “enlarged” Thymus Gland

In the first half of the 19th century, physicians were becoming alarmed by sudden infant death syndrome (SIDS). Healthy infants would be put to bed and found dead in the morning. In 1830, pathologists noted that SIDS-affected infants had enlarged thymus glands compared with “normal” autopsy specimens. It seemed logical to conclude that these “enlarged” glands were in some way responsible for the deaths.

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If an enlarged thymus was leading to sudden infant death, removal of the thymus might be of preventive value. Radiology had advanced to the point at which physicians began making the diagnosis of thymic enlargement from x-ray films. After radiographic diagnosis, thymectomy was initially recommended, but the mortality rate was unacceptably high. Thymus irradiation became the treatment of choice.

The first “successful” use of irradiation to shrink the thymus was reported by Friedländer in 1907. Thousands of children eventually received radiation to prevent status thymicolymphaticus. Some physicians advocated prophylactic irradiation for all neonates.

There was only one slight problem. It turned out to be deadly

The cadavers used by anatomists to determine the “normal” thymus size were from the poor, most having died of highly stressful chronic illnesses such as tuberculosis, infectious diarrhea, and malnutrition. What was not appreciated at the time was that chronic stress shrinks the thymus gland. The “normal” thymus glands of the poor were abnormally small. Here is where the fatal mistake occurred: because the autopsied thymus glands of the poor were regarded as normal in size, the SIDS-affected infants were erroneously believed to have thymic enlargement

In a household or community where large numbers of people express the same symptoms, it is very possible for people to assume this condition to be the normal one and good health to be the abnormality. They may even conclude healthy people are in need of treatment to bring them back to "normal" patterns of behavior.

You can see this error repeated historically, from abusive parents assuming "being beaten by my parents toughed me up so I should do the same" to anti-vaxxers who think measles and whooping cough build character.