On page 118-120, Moalem talks about how evolution has shaped many of the common infectious diseases many humans experience today using virulence, the degree to which an organism destroys its host. Moalem talks about how some diseases face evolutionary pressure against virulence, and how some diseases are selected for if they do tons of damage to their host. For example, a common cold has microbes that rely on hosts to move around and spread it that way. For that reason, a cold virus will leave a human healthy enough to move around, but just while sneezing and coughing. The cold virus needs a human to move around to spread, and according to Paul Ewald, the cold virus will never evolve to kill a host. However, malaria is selected for if it incapacitates its host, because it makes the host vulnerable to mosquitoes which spread the disease. This shows Big Idea #1(The process of evolution drives the diversity and unity of life). The process of evolution makes it so the cold virus has low virulence today, and it doesn't incapacitate a host like malaria does because they both evolved since malaria has mosquitoes to help it spread.
Find another disease and say if it has high or low virulence. Also, say if evolution will favor the disease if it exploits its host and how the disease usually spreads. Also, explain the advantages and disadvantages of having high and low virulence. Relate your answers to Big Idea #1 (The process of evolution drives the diversity and unity of life).
(Posted by Eric Huang, ehuang4@students.d125.org)
Another disease that exhibits high virulence is the virus rabies. Rabies is one of the most common neurovirulent pathogen, meaning that the disease-causing agent is capable of invading an organism's central nervous system.
ReplyDeleteEvolution favors the disease as symptoms of rabies include anxiety, confusion, agitation, and paranoia. This variety of symptoms is advantageous in that they allow for the infected organism to become more vicious and easily enraged. When the organism is in this agitated state, it tends to attack anything that slightly provokes it. This is the method in which rabies is usually spread, where a diseased organism infects another by biting.
However, there are also some disadvantages of rabies being highly virulent. Other symptoms often include paralysis and delirium, which may incapacitate the organism and prevent it from infecting others by disabling all movement. But the aggressive state that results from high virulence is much more advantageous, and thus is selected for it.
This concept relates to Big Idea 1 in that "individuals with more favorable variations... are more likely to survive and produce more offspring, thus passing traits to future generations." The high virulence of rabies allows for the infected organism to act in a way such that other organisms will easily become infected, and thus is advantageous in surviving and reproducing.