The Atlantic slave trade brought an influx of diseases, particularly malaria and yellow fever, to the Caribbean. Malaria and yellow fever were already rampant in Africa, and years of exposure in Africa rendered a great number of the incoming slaves immune to the two diseases, while others were carriers for the diseases. The arriving Europeans brought slaves to the Caribbean islands, both of which were carriers of diseases.
The population of the Caribbean is estimated to have been around 750,000 immediately before European contact, although lower and higher figures are given. After contact, social disruption and epidemic diseases such as smallpox and measles (to which they had no natural immunity) led to a decline in the Amerindian population. From 1500 to 1800 the population rose as slaves arrived from West Africa such as the Kongo, Igbo, Akan, Fon and Yoruba.
Malaria was spread by the Anopheles mosquito and yellow fever was spread by the Aedes aegypti mosquito. Mosquitoes would ingest blood from one human who was a carrier of the disease and pass the virus to another human. The presence of mosquitos to act as a vector, combined with a reservoir of infected humans, almost assured the likelihood that people in these areas would be exposed to the diseases.
While the Africans were genetically protected, the Europeans were not. Many Europeans living in the new lands would contract the diseases and die. The resistance of Africans to these diseases, which allowed them to survive and work in infested areas where Europeans couldn't, ironically increased their usefulness there and caused increased slave trade. The introduction of these two diseases into the Caribbean changed the ethnic makeup of the area, and decimating the indigenous population.