Complex sentence: During a two-year period, 50,000 of Marseille's total population of 90,000 died, and an additional 50,000 people succumbed as the plague spread north, eventually reaching Aix-en-Provence, Arles, Apt and Toulon.
Split into simpler sentences: During a two-year period, 50,000 of Marseille's total population of 90,000 died. An additional 50,000 people in other areas succumbed as the plague spread north, eventually reaching Aix-en-Provence, Arles, Apt and Toulon.