Logging, whether legal or illegal, causes much deforestation. It has the capability of displacing certain microorganisms in a particular habitat. There is mutual existence in the forest between microorganisms and other species but this relationship has been altered due to deforestation creating some occurrences of new infective diseases and the re-emergence of the old pathogens with greater resistance and impunity.
Some of man's activities contribute in decimating forests. Aside from logging, man clears land for agricultural food and non-food crops. Other factors that complement rapid deforestation include fuelwood gathering, charcoal making, mining, clearing of large tracts of forest ground for human habitation, and clearing of forest floor for water storage and dams facilities. With such activities, man has also created some man-made sites, holes, patches with stagnant water favoring some insect vectors.
Such forest disturbance could benefit more the insect vectors. In Tanzania, for instance, malaria carrier Anopheles gambiaae found a nice breeding ground in shallow patches, pits, sewers, and holes resulting from deforestation.
In 1960 in the Amazonian Brazil, malaria was declared as under control. After two decades, the disease however, re-appeared in greater proportion. Health authorities cited some factors that contributed to the new outbreak: human habitation into the rainforest, which provided the insects with new non-immune hosts, and the resulting man-made breeding sites with standing water. The same incident was observed along the TransAmazon Highway where more people contracted malaria due to increasing immigration and forest settlement.
Before deforestation, forest floor are naturally littered with organic layers (leaves, branches, and the like). This makes it quite acidic. Upon clearing or deforesting, the same piece of forest floor, now with man-made pools, is penetrated by sunlight making it warmer and at the same time rendering it neutral in pH. This condition greatly favors certain mosquito larvae to develop and multiply in enormous number.
To have malaria, three conditions must be met: the presence of human hosts; the sufficient number of malaria transmitting anopheline mosquitoes; and, the favorable conditions such as temperature and humidity, for the complete development of the parasite in the infected mosquitoes.
Some apparently healthy blood transfusion donors may also transmit the disease if parasites are not fully eradicated in him. So far, four species of malarial parasites are recognized as pathogenic to man: Plasmodium falciparum, P. vivax, P. malariae, and P. ovale. They produce different clinical fever symptoms.
Malaria has an interesting story. The early Romans thought that the disease was caused by foul air rising from the marshes. And so they drained the marshes to reduce the breeding places of mosquitoes. In so doing, unintentionally, they also reduced the occurrence of malarial infestation.