Complex sentence: The Great Plague killed over 100,000 people, about 20% of London's population and was caused by the "Yersinia pestis" bacterium which was transmitted through the bite of an infected flea, the prime vector for Y. pestis.
Split into simpler sentences: The Great Plague killed an estimated 100,000 people, about 20% of London's population. Bubonic plague is a disease caused by the "Yersinia pestis" bacterium which is usually transmitted through the bite of an infected flea, the prime vector for Y. pestis.