Life Cycle of Onchocerca volvulus:
The life cycle of O. volvulus begins when a female black fly takes a blood meal. O. volvulus microfilariae, found in the dermis of the human host, is ingested by the female fly. The microfilariae then penetrates the gut of the fly and migrates to the thoracic flight muscles where it enters the J1 larval stage. Once the microfilariae matures into the J2 larval stage, it migrates to the proboscis of the fly and resides in the saliva. Here the O. volvulus matures into the J35 larval stage and passes from the saliva of the fly to the blood of a human through the fly's pentration of the human's epidermis. The larvae then migrates to the subcutaneous tissue, forming a nodule that matures into an adult worm over a period of six to twelve months. The small adult male then migrates to the subcutaneous tissue to mate with a large adult female. The adult female usually produces 1,000 to 3,000 microfilariae per day. The eggs mature to the first stage of the microfilariae and are released into the subcutaneous tissue. The microfilariae are then taken up by a female black fly during its "blood meal," mature over a one to three week period, and the O.volvulus life cycle begins again. The life span of the microfilariae is usually one to two years and the life span of the adult worm can be up to fifteen years. 
O. volvulus cannot survive in the outside environment; therefore it constantly remains within a host, whether it be the black fly or a human. This parasite causes no harm to the intermediate host and only causes harm to its primary host after the parasite's death. Upon dying the parasite causes inflammation in various areas of the human body, producing nodules that protrude from the skin. This inflammatory response also causes damage to the optical nerves, causing blindness, which is the origin of the disease's name. As previously stated, there is no preventative vaccine or cure for the disease caused by this parasite, but medicine can be taken to temporarily alleviate the pain. 
An example of Onchcerca volvulus tranmission and relation to slavery:
O. volvulus is transmitted from human to human through the bites of black flies. Although the majority of the infections still occur in Western and Central Africa, the parasite and the debilitating disease it causes has migrated to other countries, such as those located in South and Central America. This migration is thought to be the result of the slave trade from Africa to these countries in South and Central America.  Other diseases that came over with slavery killed many slaves before landing in the New World, reducing the spread and effects of those diseases;however, the disease caused by the aforementioned parasite is not fatal so it was successfully transferred to inhabitants of the New World, causing this debilitating disease to spread to those who forced the Africans into slavery. So, while this parasite is certainly not the cause of the slave trade from Africa, it could be seen as a form of unintentional retaliation from those forced into slavery, and an example of how a small parasite can somewhat alter society and history.