Tropical and sub-tropical regions are the main areas affected by nematodes and parasitic worms, which often causes filariasis. Around 20% of immigrants to Spain are children from these regions. There are concerns about absolute eosinophilia in immigrants that is correlated with parasitic diseases that may go undiagnosed. Absolute eosinophilia is clinically diagnosed as >0.45×10 eosinophilic leucocytes/L of peripheral blood. Recent studies suggest that around 60% of children with relative eosinophilia contracted this via parasitic infections. Relative eosinophilia is different from absolute because relative refers to an increase in percentage of white blood cells (i.e. leukocytes) due to a loss of blood plasma; where as absolute eosinophilia is purely an increase in white blood cell production. Of those with relative eosinophilia, 40% were undiagnosed until these studies. Therefore, there is a great need for thorough parasitological studies in this area of tropical infectious diseases.

Acanthocheilonemiasis is caused by the parasite, "mansonella perstans." "M. perstans" is primarily found in central Africa and in some areas of South America, therefore the most affected populations are located in these areas. Acanthocheilonemiasis affects humans in these areas in equal numbers. The prevalence of this condition does significantly increase with age. Furthermore, the parasite is most commonly found in areas of tropical forests with alternating swamps and open ground.

Approximately 114 million people in Africa are infected with "M. perstans", including 33 sub-Saharan African countries. Recent studies focused on Gabon specifically, where febrile and tropical diseases are common. Contrary to popular recent suggestions, "M. perstans" does not influence the emergence of febrile diseases, including HIV, tuberculosis, bacteremia, and malaria. In general, hemoglobin levels in individuals with malaria are severely reduced from that of a healthy individual. Reduced levels occur because the malaria parasite, "Plasmodium falciparum," utilizes human hemoglobin as its major energy source. Filariasis, in combination with severe malaria, actually shows higher hemoglobin levels than in severe malaria alone. In addition, "M. perstans" did not have adverse effects on those with HIV, as there were actually higher levels of CD4 in HIV patients co-infected with "M. perstans". Further research in this area may allude to clinical manifestations of this infectious disease, as there could be possible benefits by contracting "M. perstans".