A believer in the miasma theory of disease, Whitehead worked to disprove false theories, eventually coming to prefer John Snow's idea that cholera spreads through water contaminated by human waste. Snow's work — and Whitehead's own investigations — convinced Whitehead that the Broad Street pump was the source of the local infections. Whitehead then joined with Snow in tracking the contamination to a faulty cesspool and the outbreak's index case.
Whitehead's work with Snow combined demographic study with scientific observation, setting important precedent for the burgeoning science of epidemiology.
Whitehead served in several other London parishes before moving to Newlands in Cumberland in 1884, finally becoming vicar of Lanercost for five years until his death.