Economic development improves long-run health outcomes through access to medical treatment, sanitation, and higher income. Short run impacts, however, may be ambiguous given disease exposure from market integration. Using a panel dataset of Japanese vital statistics and multiple estimation methods, I find that railroad network expansion is associated with a six percent increase in gross mortality rates among newly integrated regions. Communicable diseases accounted for most of the rail-associated mortality, which indicate railways behaved as transmission vectors. At the same time, market integration facilitated by railways corresponded with an eighteen percent increase in total capital investment nationwide over ten years.