Editing Pathogen transmission (section)

==Relationship with virulence and survival==

Pathogens must have a way to be transmitted from one host to another to ensure their species' survival. Infectious agents are generally specialized for a particular method of transmission. Taking an example from the respiratory route, from an evolutionary perspective viruses or bacteria that cause their host to develop coughing and sneezing symptoms have a great survival advantage, as they are much more likely to be ejected from one host and carried to another.  This is also the reason that many microorganisms cause [[diarrhea]].{{cn|date=June 2021}}

The relationship between [[virulence]] and transmission is complex and has important consequences for the long term evolution of a pathogen. Since it takes many generations for a microbe and a new host species to co-evolve, an emerging pathogen may hit its earliest victims especially hard. It is usually in the first wave of a new disease that death rates are highest. If a disease is rapidly fatal, the host may die before the microbe can be passed along to another host. However, this cost may be overwhelmed by the short-term benefit of higher infectiousness if transmission is linked to virulence, as it is for instance in the case of cholera (the explosive diarrhea aids the bacterium in finding new hosts) or many respiratory infections (sneezing and coughing create infectious [[aerosol]]s).{{cn|date=June 2021}}

Anything that reduces the rate of transmission of an infection carries [[externality|positive externalities]], which are benefits to society that are not reflected in a price to a consumer.  This is recognized implicitly when vaccines are offered for free or at a cost to the patient less than the purchase price.<ref>{{cite journal | vauthors = Graves S, Samuelson DA | title = Externalities, public goods, and infectious diseases. | journal = Real-World Economics Review | date = March 2022 | volume = 99 | pages = 25–56 | url = http://www.paecon.net/PAEReview/issue99/GravesSamuelson99.pdf }}</ref>