Complex sentence: When dapsone was used alone, the "M. leprae" population quickly evolved antibiotic resistance; by the 1960s, the world's only known anti-leprosy drug became ineffective against resistant bacteria.
Split into simpler sentences: When dapsone was used alone, the "M. leprae" population quickly evolved antibiotic resistance. By the 1960s, the world's only known anti-leprosy drug became ineffective against resistant bacteria.