Friday, February 14, 2014

Pathogenicity and Virulence

A Reoccurring Concern

Spanish Flu, Bird Flu, Pig Flu, Ebola(diseases). Every so often it seems as if a new disease is going to be the next pandemic, with the exception of the Spanish flu that proved to be a pandemic. Somehow many of us survive to face the next one. Conspiracy theorists say the government is behind it. Though there is possibility that they are right, the correct answer lies within a deeper understanding of how pathogens function. The pathogen’s ability to infect and the magnitude of the infection play a significant role in its success. When a pathogen finds the perfect balance, it will be very successful. [5] 

The Difference 

A pathogen is defined as a microorganism that causes disease. The pathogen has two important characteristics. The first is pathogenic character, which is defined as the pathogen’s ability to cause disease in its host. The second characteristic is virulence, which is defined as the magnitude at which the disease occurs. Pathogenicity is a quality that is either on or off. The virulence factor measures the level of pathogenicity. There are multiple virulence factors, such as attachment, destructive enzymes and toxins. Virulence factors allow for the pathogen to be more successful within a limit. To explain, if a pathogen has zero virulence then it is no longer actually a pathogen by definition because it is not causing disease. On the other hand, if the virulence factors are too high then the pathogen will be so efficient at killing its host that it may deplete its reservoir, the host it infects, and no longer reproduce.

Chestnuts No More

Chestnut trees(host) were one of the most utilized trees in America. It was used for the durability of its bark and the tastiness of its chestnuts. However, this only lasted up until the 1900.This is when the Chestnut blight took place. A fungus, Cryphonectria parasitica that originated in China and only infected dying twigs and pieces of bark, was introduced to America around 1893. In the fungus' native region there existed a different kind of chestnut tree which was resistant to the fungus. Once introduced t the American chestnut tree it only took 40 years to wipe out all the chestnut trees in America. Cryphonectria parasitica spread up to 50 miles of chestnut forestry a year. Once the chestnut trees were decimated the fungus' reproduction rate plummeted, without a host to provide the nutrients and a habitat it's reproduction was halted. [2] [3]

No Need to worry

The perfect balance the pathogen seeks to find can only be successful if they successfully infect their host. If the pathogen is successful in reaching a balance, there is a multitude of ways that interaction with a pathogen can be avoided. If interaction still occurs your body has mechanisms that are in place to deal with the pathogen. If the mechanisms fail the human population is a genetically diverse species and a resistant human can exist and survive. The host and pathogen have an eternal struggle that is an evolutionary arms race. The victory has yet to be decided.



  1. Another example was the Myxoma virus that was used to control rabbit populations. At first the virus worked, but later on it became less effective. The reason for this being the highly virulent strains were able to kill of their hosts very quickly. By killing their hosts too fast, the virus is not able to spread itself to other hosts. As a result, this selection against the highly virulent strains decreased the host fitness too much. In order to rectify this issue, an optimal virulence level needs to be achieved so that the pathogen is able to survive while not causing a deadly burden on the host.