We have seen that different species of parasites have markedly different impacts upon health. Some may kill their host and so reduce the population size, others cause chronic anaemia or malnutrition and so reduce their physical productivity, while the remainder may cause symptoms that reduce psychological well-being or cause no symptoms at all. If we use disability-adjusted life years to compare the impact of parasites upon different ancient civilizations, we can estimate the degree to which each were burdened by parasite infection. Using this approach would suggest that the civilizations with the heaviest health burden from parasites were (1) Egypt and Nubia; (2) Roman Empire; (3) Ancient China, Korea, and Japan; (4) South American Pacific Coast civilizations such as Maya, Moche, and Inca; and (5) the Ancient Near East. As many parasites impair the ability of labourers to complete physically arduous work, it is possible that the need for new, healthier workers to complete national infrastructure projects may have triggered military expeditions to neighbouring states to obtain prisoners to work as slaves in many of these civilizations.