Complex sentence: A transitional period began in the late 1850s as the work of Louis Pasteur and Robert Koch provided convincing evidence; by 1880, the miasma theory was struggling to compete with the germ theory of disease.
Split into simpler sentences: A transitional period began in the late 1850s as the work of Louis Pasteur. By the end of the 1880s the miasma theory was struggling to compete with the germ theory of disease.