What is a pathogen?

I have been quite interested for some time in the nature and origin of pathogens. For a creationist who believes the original creation was benign and absent of animal and human death and suffering, pathogens have always struck me as a difficult topic. Put in more crass terms, if God is so good, why is there anthrax, AIDS or the flu?

Creationists have emphasized the idea of degeneration to explain pathogens, but that only carries you so far. It turns out that the nature of pathogens and pathology is quite a bit more complicated than what we might expect at first glance. What hasn't been nearly as emphasized in the creationist literature is the idea that the host is at least as much to blame as the "pathogen" for the origin of disease, but surely that seems like an obvious place to look for problems.

That's why I was intrigued by an interesting new Q&A essay from Pirofski and Casadevall in BMC Biology. They downplay the very idea of "pathogen" or "commensal" and instead focus on the relationships between organisms.
But surely in the case of immunity the pathogen is still a pathogen, it's just that immunity prevents you from getting sick, right?
Not really. The question implies that the ability to cause damage or disease is an inherent microbial property, but in fact these characteristics only exist in the context of a susceptible host. Therefore, when a host is immune, pathogenicity is not expressed. What is important to recognize is that pathogenicity and virulence are microbial properties that can only be expressed in a susceptible host.

It's an interesting read, and totally free. So check it out:

Pirofski and Casadevall. 2012. Q&A: What is a pathogen? A question that begs the point. BMC Biology 10:6.

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