Mechanisms of infectious disease
The fiction blog io9.com has a very accessible summary of the mechanisms by which an infectious agent (virus, bacterium, fungus, protozoan) can cause disease once it gets into the human body. Instead of reprinting the article, I will call out a set of bullet points from it, and point everyone to the original article. I urge all BIO230 students to go read this summary, as this covers a lot of the material in our upcoming exam. The author, Joseph Bennington-Castro outlines 4 basic mechanisms which allows a microorganism to cause disease once it is in the human body. Several of these have been discussed in BIO230 lecture:
- Exotoxins. These are proteins that are made by and secreted by many different kinds of microorganisms, which when released by the pathogen directly cause damage to the host. There are many, many different microorganisms that can make exotoxins.
- Endotoxins. These are lipid components of the Gram negative outer membrane, which when released into the blood stream (either by the bacterium being killed by phagocytosis, or by an antibiotic) cause a variety of disease conditions throughout the body. These symptoms include fever, inflammation, shock, and disseminated intravascular coagulation.
- Immune response. The presence of some pathogens can trigger an inappropriate immune response that itself can cause damage to the human body. Rhinoviruses for instance trigger localized inflammation in the upper respiratory tract during infection, with very little cellular damage. Symptoms of disease are therefore caused by the immune response.
- Tissue burden. The picture to the right is a Giardia lamblia cell, a protozoan parasite of the mammalian intestine. In an acute Giardia infection, the walls of the intestine are coated by the organism as it attaches using the suction cup seen in the picture. Since the intestine is coated with these cells, nutrient adsorption by the host is significantly impaired, resulting in diarrhea.