Frerichs is generally recognized as the founder of experimental pathology. As a professor of medicine for many years, his insistence on teaching medical biochemistry and pathology as part of the regular curriculum was instrumental in establishing a scientific foundation for clinical medicine.

Born in Aurich, Hanover, Germany, Frerichs received a medical education in his homeland and went on to become a professor of therapeutics and pathology at the illustrious University of Breslau in 1851. During his eight years there, he was known for his strict requirement that all his students' work be based on well designed experiments and precise laboratory analysis. One of the students whose medical career Frerichs helped shape was Paul Ehrlich, who would later win a Nobel Prize in Medicine for his research.

While he was working at Breslau, Frerichs also performed his own research on the biochemistry of diseased organisms. Through this work, he became the first scientist to observe the amino acids tyrosine and leucine in urine from hepatitis patients with acute yellow liver atrophy. He did extensive work on the pathologies of nephritis and liver cirrhosis, leading to better diagnostic and treatment methods for liver ailments. His Treatise on Diseases of the Liver was published in 1858. Frerichs's other research of note was on the pathology of diabetes.

Beginning in 1859, Frerichs served as director of the University of Berlin's Charity Hospital. He remained in that post until his death in 1885.