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Two children have fever and, a few days later, a rash. Is it the same disease? Before laboratory tests, the only way to tell was to watch closely and compare. More than a thousand years ago the physician Abu Bakr al-Razi did exactly that for smallpox and measles. This post covers who he was, what his treatise says in its own words, how it compares with the World Health Organization's descriptions today, and the reasoning habit it teaches.
Who was al-Razi?
The US National Library of Medicine's exhibition Islamic Culture and the Medical Arts gives the facts. He was born in the Iranian city of Rayy in 865 and died there about 925. Learned in philosophy, music and alchemy, he headed hospitals in Rayy and Baghdad. Europe knew him as Rhazes.
His largest work, the Hawi or Comprehensive Book on Medicine, was a private notebook of extracts from earlier writers and his own clinical cases, put in order by his pupils after his death. The NLM notes that he was willing to correct Galen from experience: on one urinary ailment he said that Galen had seen only three cases while he had seen hundreds.
What is the treatise on smallpox and measles?
The NLM describes his Kitab fi al-jadari wa-al-hasbah as influential in Europe, and twice translated into Latin in the 18th century, when inoculation was a live question. It was not the first monograph on the subject; the exhibition gives that honour to Thabit ibn Qurrah. W. A. Greenhill translated it from Arabic into English for the Sydenham Society in 1848, in A Treatise on the Small-Pox and Measles.
How did al-Razi tell them apart?
Chapter III of Greenhill's translation lists the signs before the rash. For smallpox: "a continued fever, pain in the back, itching in the nose, and terrors in sleep", along with red cheeks and eyes, heaviness, a sore throat and cough. Many signs are shared. The difference, he writes, is "that the inquietude, nausea, and anxiety are more frequent in the Measles than in the Small-Pox; while, on the other hand, the pain in the back is more peculiar to the Small-Pox than to the Measles".
In a later section he puts it more briefly: the premonitory signs of smallpox include "a violent pain in the back", but "if the fever be attended with more vehement heat, and greater anxiety, then the Measles may be expected."
How does this compare with what WHO says today?
The WHO smallpox page lists early symptoms as "high fever, fatigue and severe back pain", followed two to three days later by a rash of fluid-filled bumps that begins on the face and hands. The WHO measles fact sheet lists fever, runny nose, cough, red and watery eyes and small white spots inside the cheeks, with the rash appearing three to five days after the first symptoms, usually on the face and upper neck.
Al-Razi's key marker, back pain for smallpox, matches WHO's modern description a thousand years later.
Worked example: separating shared signs from telling signs
Sort al-Razi's list the way a clinician sorts evidence.
Shared signs, which do not help you choose: fever, red eyes, sore throat, cough, rash.
Signs that lean to smallpox: marked back pain.
Signs that lean to measles: more restlessness, nausea and anxiety; in WHO's list, a runny nose and white spots inside the cheeks.
Now a check question: a patient has fever, red eyes and cough. Which disease? You cannot say; all three are shared. Add severe back pain and the balance tips one way; add white spots inside the cheeks and it tips the other. The habit is to ask of every sign, "would I expect this in the other disease too?"
Which mistakes does this help you avoid?
Treating every symptom as evidence. A sign common to both diseases cannot distinguish them, however striking it is.
Reading history as current medicine. Smallpox no longer occurs: WHO records the last natural case in Somalia in 1977 and the declaration of eradication in 1980. Measles still kills; WHO estimates 95,000 deaths in 2024, mostly among unvaccinated young children.
Our post on Edward Jenner and the first vaccine picks up the story of how smallpox was beaten, and Ibn Sina and the Canon of Medicine covers the next great Persian physician.
What can a student learn from al-Razi?
He trusted careful, repeated observation over authority, and he wrote down which features separate two similar things. That is exactly the skill tested in biology questions that ask you to distinguish two processes, tissues or diseases.
When two biology ideas blur together, the Fermi Concept Library covers every concept in biology, so you can compare them side by side.
Try it now: pick two easily confused topics, such as mitosis and meiosis, and list their shared and telling features in two columns.
