This spice/herb had a strong reputation for its healing properties during the middle ages, and was even used to help prevent the plague.

The spice/herb you are referring to is Sage. During the Middle Ages, sage was highly regarded for its medicinal properties and was believed to have various health benefits, including the ability to ward off illnesses such as the plague. While other herbs like cilantro and black pepper have their uses, sage is the one most historically associated with healing and protection during that time.