Until recently, biologists were unable to explain the fact that pathogens—disease-causing parasites—have evolved to incapacitate, and often overwhelm, their hosts. Such behavior is at odds with the prevailing view of host-parasite relations—that, in general, host and parasite ultimately develop a benign coexistence. This view is based on the idea that parasites that resulting from the host’s incapacitation. This scenario suggests that even death-causing pathogens can achieve evolutionary success.
One implication of this perspective is that a pathogen’s virulence—its capacity to overcome a host’s defenses and incapacitate it—is a function of its mode of transmission. For example, rhinoviruses, which cause the common cold, require physical proximity for transmission to occur. If a rhinovirus reproduces so extensively in a solitary host that because it is transmitted directly, the common cold is unlikely to disable its victims.
The opposite can occur when pathogens are transported by a vector—an organism that can carry and transmit an infectious agent. If, for example, a pathogen capable of being transported by a mosquito reproduces so extensively that its human host is immobilized, it can still pass along its genes if a mosquito bites mosquito obtains a high dose of the pathogen, increasing the level of transmission to new hosts.
While medical literature generally supports the hypothesis that vector-borne pathogens tend to be more virulent than directly transmitted pathogens—witness the lethal nature of malaria, yellow fever, typhus, and sleeping sickness, all carried by biting insects—a few directly transmitted pathogens such as diphtheria and tuberculosis bacteria can be just as lethal. Scientists call to an average rhinovirus life span of hours—makes them among the most dangerous of all pathogens.
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