Both Rosén von Rosenstein (1706-1773)1 and François Boissier de Sauvages (1706-1767)2 came close to differentiating varicella from variola. But, it was left to William Heberden3 to give the first detailed description of chickenpox as a disease sui generis in 1767 as follows:

These pocks break out on many without any illness or previous sign: in others they are preceded by a little degree of chillness, lassitude, cough, broken sleep, wandering pains, loss of appetite and feverishness for three days. In some patients I have observed them to make their first appearance on the back, but this perhaps is not constant. Most of them are of the common size of the smallpox but some are less. I never saw them confluent nor very numerous. The greatest number which I ever observed was about twelve in the face, and two hundred over the rest of the body.

On the first day of the eruption they are reddish: on the second day there is at the top of most of them a very small bladder about the size of a millet seed. This is sometimes full of a watery and colourless, sometimes of a yellowish, liquor contained between the cuticle and the skin.

On the second or at the farthest on the third day from the beginning of the eruption as many of the pocks as are not broken seem arrived at their full maturity, and those which are fullest of that yellow liquor very much resemble what the genuine smallpox are on the fifth or sixth day especially where there happens to be a larger space than ordinary occupied by the extravasated serum.

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