Complex sentence: A former believer in the miasma theory of disease, Whitehead worked to disprove false theories, eventually focussing on John Snow's idea that cholera spreads through water contaminated by human waste.
Split into simpler sentences: A former believer in the miasma theory of disease, Whitehead worked to disprove false theories. He was influenced by Snow's idea that cholera spreads by consumption of water contaminated by human waste.