A parasite, as defined by the Merck Manual, is a type of organism that lives inside a host, human or otherwise, and causes harm to the host. These parasites can live anywhere inside the human body. More commonly, human parasites live in the blood or the intestines in order to obtain a rich nutrient supply. Parasites enter the body in several different ways and more commonly affect people living in rural and developing areas.
Ingestion
Contaminated food or water is ingested, which allows for parasites to set up inside the colon. It's often difficult to see the parasites in the food. Often raw and undercooked food, specifically fish and red meat, contains eggs and larvae of parasitic worms. Once inside the body, these parasites begin to hatch and develop while traveling through the digestive system. Eventually they attach to the intestinal walls and continue to grow. Adult forms of parasitic worms can be seen with the naked eye, according to the Centers for Disease Control and Prevention, or CDC. Water can be contaminated with protozoa, or single-celled organisms. Once the water is ingested, these single-celled parasites are able to replicate inside the body. Giardia and toxoplasma are the two most common water- and food-borne illnesses in the United States, according to the CDC, with 2 million and 1.5 million cases annually, respectively.
Insect Bites
Beetles, mosquitoes and flies can all carry parasites in their systems and transmit them to humans through their bite or their dung getting into an open wound on the skin. Any organism that carries a disease and can transmit it to another is called a vector. These insects carry various parasites transmitted through the saliva and penetrate the skin barrier once the insect bites and creates a wound. Malaria, which is transmitted by specific species of mosquitoes, is the No. 1 parasitic killer worldwide with about 3 million reported cases and more than 1 million deaths annually, according to the CDC and World Health Organization.
Skin Absorption
Some species of parasites are so small they can find their way into the body through the pores of intact skin. The roundworm, or nematode, is just such a parasite. Walking barefoot through infected fields or beaches will allow these parasites to enter the body. From there they go through a complex life cycle within the body. These parasites are more common in tropical, subtropical and sometimes temperate areas, according to the CDC. Additionally, the CDC states, these parasites more commonly affect rural, underdeveloped nations and lower socioeconomic groups.
