Complex sentence: In 1546 Girolamo Fracastoro (1478--1553) wrote a classic description of measles and thought that the disease was caused by "seeds" ("seminaria") that were spread from person to person.
Split into simpler sentences: In 1546 Girolamo Fracastoro (1478--1553) wrote a classic description of measles. He believed that the disease was caused by "seeds" ("seminaria") that were spread from person to person.