UCLA

In the centuries following Christopher Columbus’s 1492 journey to the Americas, transoceanic voyages opened unprecedented pathways in global pathogen circulation – commonly termed the “Columbian Exchange”. Yet no biological transfer is a single, discrete event. We use the- oretical modeling to quantify historical risk of shipborne pathogen introduction, exploring the respective contributions of journey time, ship size, ship susceptibility, transmission inten- sity, density dependence, and pathogen biology. We contextualize our results using arrivals data from San Francisco Harbor, 1850–1852, and from a selection of historically significant voyages, 1492–1918. We offer numerical estimates of introduction risk across historically- realistic journey times and ship population sizes, and show that steam travel and shipping regimes which involved frequent, large-scale movement of people both substantially increased risk of transoceanic pathogen circulation.