Here is a more detailed description of what Chibi Romano actually suffered from in accordance to canon and with actual medical research. (by fanfiction member Tawariell Saerwen)
Chibi Romano suffered from Chorea, or specifically Sydenham’s chorea. It is also known as St. Vitus Dance, named after the Saint who also was afflicted by this disorder. This is a symptom that develops after a patient has suffered from acute rheumatic fever. Rheumatic fever is caused by a Streptoccocus pyogenes infection, or in layman’s terms, strep throat. Strep infections are treatable with antibiotics; patients who deny antibiotics end up developing rheumatic fever or worse – Necrotizing fasciitis, also known as flesh-eating disease. If a patient has a high temperature, they’re at risk of developing rheumatic fever. If a patient has the chills and can’t warm up, they’re at risk of developing flesh-eating disease.
During the Renaissance, there was no medicine to cure diseases. Germs had not been discovered and cleanliness in Europe was poor. If somebody were ill, it was usually believed to be the cause of demon possession or simply God’s Will. So, in this case, how could Chibi Romano have survived after suffering from acute rheumatic fever? The answer would be his immune system and natural selection. His immune system would have to been very strong and resistant toward the bacterial infection and going into overdrive to beat the disease. Because of this, his central nervous system would end up becoming chemically over stimulated and his limbs would begin to involuntarily jerk and twitch, thus developing symptoms of Sydenham’s chorea, the cause of his clumsiness.
The good news is that Sydenham’s chorea eventually clears up on its own and is treatable and curable through physical therapy. So with accordance to canon, dancing the Tarantella cured Chibi Romano’s chorea symptoms.
The bad news is that, unfortunately, Chibi Romano would have developed permanent damage to his body such as carditis (inflammation of the heart) and Rheumatoid arthritis (inflammation of the joints) after surviving from acute rheumatic fever.