Rhazès
The post-antique European world gave rise to a dominant monotheistic culture and with it a de facto ban on human dissection. As such, there was a slow down in comparative medicine's progress through the middle ages. This was to be codified in 1637 CE with René Descartes manuscript "Discourse on the Method". The Persian physician Muhammad ibn Zakariya al-Razi (865 - 925 CE) was the first to describe smallpox and measles and prescribe treatments, making his discoveries largely through animal dissection.