Complex sentence: When a man would get this disease, they only way they believed a man could rid himself of painful sores was to pass it to a woman, the reasoning being that a woman's body can absorb infection and purge it during a cycle.
Split into simpler sentences: When a man would get this disease, the only way they believed he could rid himself of painful sores was to pass it to a woman. The reasoning was that a woman's body can absorb infection and purge it during a cycle.