Besides being somewhat terrifying and fascinating at the same time: infectious disease and parasitism. Not all species of  insects, bacteria, and crustaceans are parasites, but there are species in each group that fill that role.

Infectious diseases are caused by external sources such as a parasite, fungus, or a virus. You might wonder, “Ok, I know what a fungus and virus is, but what exactly constitutes a parasite? What organisms are included when you say parasite?”  A parasite is any living organism that lives in or on another organism (referred to as a host) and derives essential nutrients from that host. Therefore, a parasite could be anything from a microscopic organism to a tick that embeds under the skin and drinks blood for a few days to the crustacean that replaces a fish’s tongue.

So how does a host become infected with a parasite, especially one that causes disease? Transmission occurs in a variety of ways, and the methods people are most familiar with is direct contact, droplet contact, or through the air.

• Direct contact would be through physical contact between two individuals including contacting bodily fluids of an infected individual. This method is the most common way Ebola is transmitted.

• Droplet contact is the most common way flu is spread and occurs when the virus is found in droplets produced by sneezing and coughing.

• Airborne transmission occurs when microbes can survive for long periods of time outside of the body and are resistant to drying. Some examples of diseases caused by viruses or parasites transmitted this way are tuberculosis and chickenpox. 

 

One form of transmission that is becoming more well known, involves transmission of an organism or virus through way of another organism (a vector). Transmission from a vector species (examples include mosquitoes, fleas, ticks, and flies) usually occurs through a bite. Parasites and viruses that are spread this way cause diseases referred to as vector-borne diseases. Some examples of vector-borne diseases are Lyme disease, Dengue, Malaria, the Plague, Rocky Mountain spotted fever, and Chaga’s disease.

 

Vector transmission is somewhat more complicated than the other forms of transmission because it involves aspects of both the vector species and the organism or virus that is being transmitted. In order to fully understand the ecology and evolution of the parasite of virus, it is important to also understand the ecology and evolution of the vector species.

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Pictures Courtesy of:

“Live Tetragnatha montana (RMNH.ARA.14127) parasitized by Acrodactyla quadrisculpta larva (RMNH.INS.593867) – BDJ.1.e992″ by Miller, J. A.; Belgers, J. D. M.; Beentjes, K. K.; Zwakhals, K.; van Helsdingen, P. – Miller, J. A.; Belgers, J. D. M.; Beentjes, K. K.; Zwakhals, K.; van Helsdingen, P. (2013). “Spider hosts (Arachnida, Araneae) and wasp parasitoids (Insecta, Hymenoptera, Ichneumonidae, Ephialtini) matched using DNA barcodes”. Biodiversity Data Journal 1: e992. DOI:10.3897/BDJ.1.e992.. Licensed under Creative Commons Attribution 3.0 via Wikimedia Commons – https://commons.wikimedia.org/wiki/File:Live_Tetragnatha_montana_(RMNH.ARA.14127)_parasitized_by_Acrodactyla_quadrisculpta_larva_(RMNH.INS.593867)_-_BDJ.1.e992.jpg#mediaviewer/File:Live_Tetragnatha_montana_(RMNH.ARA.14127)_parasitized_by_Acrodactyla_quadrisculpta_larva_(RMNH.INS.593867)_-_BDJ.1.e992.jpg

“Cymothoa exigua parassita Lithognathus mormyrus” by Marco Vinci – Own work. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons – https://commons.wikimedia.org/wiki/File:Cymothoa_exigua_parassita_Lithognathus_mormyrus.JPG#mediaviewer/File:Cymothoa_exigua_parassita_Lithognathus_mormyrus.JPG

“Schistosoma mansoni2″ by Original uploader was Waisberg at en.wikipedia – Transferred from en.wikipedia; Transfer was stated to be made by User:Gliu.Davies Laboratory Uniformed Services University Bethesda, MD dead linkInformation presented on USUHS web site is considered public information and may be distributed or copied. Use of appropriate byline/photo/image credits is requested.. Licensed under Public domain via Wikimedia Commons – https://commons.wikimedia.org/wiki/File:Schistosoma_mansoni2.jpg#mediaviewer/File:Schistosoma_mansoni2.jpg

“Borrelia burgdorferi (CDC-PHIL -6631) lores” by Photo Credit:Content Providers(s): CDC – This media comes from the Centers for Disease Control and Prevention’s Public Health Image Library (PHIL), with identification number #6631.Note: Not all PHIL images are public domain; be sure to check copyright status and credit authors and content providers.English | Slovenščina | +/−Cropped and uploaded originally to (https://en.wikipedia.org/wiki/Image:Borrelia_image.jpg). Licensed under Public domain via Wikimedia Commons – https://commons.wikimedia.org/wiki/File:Borrelia_burgdorferi_(CDC-PHIL_-6631)_lores.jpg#mediaviewer/File:Borrelia_burgdorferi_(CDC-PHIL_-6631)_lores.jpg

“Ixodes scapularis” by Content Provider(s) : CDC/ Michael L. Levin, Ph. D. Photo Credit : Jim Gathany – This image is in the public domain : https://phil.cdc.gov/phil/detail.asp?id=1669 Transferred from fr.wikipedia. Licensed under Public domain via Wikimedia Commons – https://commons.wikimedia.org/wiki/File:Ixodes_scapularis.jpg#mediaviewer/File:Ixodes_scapularis.j