An important monograph on fevers.

Sennert gave the first scientific description of scarlet fever. He was the first to mention the scarlatinal desquamation, the early arthritis, and post-scarlatinal edema, but made no mention of sore throat.

Besides giving early accounts of scarlatina and rubella, Sennert added to the knowledge of scurvy, dysentery and alcoholism. He was an able clinician and also a believer in witchcraft. His Opera was first published in 1641; the edition given above is regarded as the best.