The historian Adam Hochschild argued that the dramatic fall in the Free State population was the result of a combination of "murder", "starvation, exhaustion and exposure", "disease" and "a plummeting birth rate".[42] Although it is impossible to be sure in the absence of records, violence and murder represented only a fraction of the total. In a local study of the Kuba and Kete peoples, the historian Jan Vansina estimated that violence accounted for the deaths of less than five percent of the population.[43]
Diseases imported by Arab traders, European colonists and African porters ravaged the Congolese population and "greatly exceeded" the numbers killed by violence.[44] Smallpox, sleeping sickness, amoebic dysentery, venereal diseases (especially syphilis and gonorrhea) and swine influenza were particularly severe.[45] Sleeping sickness, in particular, was "epidemic in large areas" of the Congo and had a high mortality rate.[46] In 1901 alone, it is estimated that as many as 500,000 Congolese died from sleeping sickness.[47] Vansina estimated that five percent of the Congolese population perished from swine influenza.[48] In areas in which dysentery became endemic, between 30 and 60 percent of the population could die.[49] Vansina also pointed to the effects of malnutrition and food shortages in reducing immunity to the new diseases.[43] The disruption of African rural populations may have helped to spread diseases further.[39]
It is also widely believed that birth rates fell during the period too, meaning that the growth rate of the population fell relative to the natural death rate. Vansina, however, notes that precolonial societies had high birth and death rates, leading to a great deal of natural population fluctuation over time.[50] Among the Kuba, the period 1880 to 1900 was actually one of population expansion.[44]