Whenever a name is common, many people are unaware of the different routes it might have come through to be named. Chicken pox is one such common disease today where few people know exactly how it came to be named. For those with a feeling for a bit of medical sleuthing through history, there are currently two theories that could account for it.
Europe is where the documentation begins on chicken pox, but it was called by many different names. Varicelle is the French term, and Windpocken is the name Germans gave it. No matter what country it was in, the results were the same. It was a disease associated with fever and blisters that would erupt on the skin. Each country use their own language to name it, but the common name today is in English.
While it was originally diagnosed by Dr. Filippo in Italy a century earlier, Dr. Morton of England associated it with small pox in the fifteenth century. He declared it was a milder variation of that now defunct disease, but his assessment was that this milder form was still a pox. It might have been helpful because the two diseases appeare to look the same, but they are not actually related.
Fast forward about a century, and there is the famous Dr. Samuel Johnson who was the next physician involved in the name. He accepted the theory this virus was a lesser form of small pox, and his use of common English gave it the name chicken. It was a common derivative of the word coward at the time, and the naming theory says that it encompasses the fact that chickenpox is much less dangerous than smallpox.
This thread of theories works well, but yet another English physician enters the picture in the late eighteenth century. Chickenpox and smallpox are not actually related viruses, and Dr. Heberden was the one to bring this to the attention of the medical world. This is where the second theory of how chickenpox was named came into being.
As a rule, itching is one of the most common symptoms of this virus, and the second theory is centered on that one part of the disease. As part of the word usage at the time, Giccan is the Middle English word for itching, and allegedly the name stuck when it was turned into the word chicken. As for pox, it has always been used to describe blisters associated with this and other viruses, and it was thought to be appended on to the name as yet another descriptive term.
Many people have come up with their own theories of how this virus was named, and it is true there is little enough documentation to settle any one claim. One of the most common thoughts is that the blisters associated with it look like a person had been attacked by chickens, but that is not part of any documentation by physicians or the world of medicine. For those seeking more information, they might only be able to find it if they invent a time machine and visit the distant past.