Evolution of Infectious DiseasePaul Ewald’s book is a rant against conventional wisdom. It opens with a flat out denial: parasites and diseases do not tend to evolve toward more benign relationships with their hosts. The conventional wisdom is based a series of just so stories, an optomism that would do Pangloss proud and a Kansas school board model of evolution.

This stuff has consequences. It’s actionable. Bacterial, viruses, etc. evolve very quickly. When we change their environment they are almost always find ways to leverage those changes. Decisions about public health can leverage that in positive ways. Or, they can blindly ignore it and creating horrible unintended consquences.

I read, rather than skimmed, the whole book because the stories are rich in analogies to the stories I’m most interested in: those about middlemen, platforms, and networks. For example mosquitos act as a intermediary for malaria. In this domain they are called the “vector.” Like the postal system, UPS, or the rail roads they provide transportation services. Understanding many of the stories in this book demands picking apart how how the infecious agent evolves in the face of pressure from both the distribution channel and the host. If the nature of one changes the bacteria (virus, tape worm, etc.) changes to strike a different balance.

There is a facinating insight here: severity of the disease is tied to the nature of the distribution channel. For example diseases which use mosquitos for as their vector, like malaria, are usually more severe than airborne diseases. A mosquito borne disease tends to be severe so it can immobilize it’s host and assure that the mosquito has an easy time feeding and when it does it’s meal contains carries the infection. If we can arrange for the patients to be moved into well screened houses where they can’t be reached by the mosquitos then this scheme falls apart. In which case it’s preferable for the affliction to evolve to keep their hosts mobile – i.e. a less severe varient of the disease emerges. This kind of modeling suggests why the common cold is relatively benign (it needs to keep the host mobile). It is very suggestive about why the trenches and field hospitals of the first world war may have generated the 1918 influensia epidemic; where the army provided continual supply of fresh hosts and mixed them intimately with those infected.

Key to many of the interesting scenarios around networks, standard, and businesses are the situations where the links are made between two different groups and these stories with the disease is the leveraging services of two distince parties seem quite analagous. A middleman, in a business context, covers his expenses by charging the parties on either side, usually differing amounts. So the dating service will charge men more than women while eBay will charge sellers while it advertises to buyers.

The same pattern happens here. Malaria is reasonably benign from the point of view of the mosquito. Analagously the fraud around eBay, the broken hearts around dating services, the viruses on Windows, the spam in your mail box are all reasonably benign from the point of view the intermediaries.

Reading this book you begin to think that any time you see mixing between two classes of actors you need only look and you’ll find a parasite that’s discovered a way to play the middleman. The stories I found the most disturbing are the ones where caretakers become the vector. There is a disease of coconut palms that uses the machette’s of the plantation workers as it’s vector. That story has the horrible plot twist that there are two ethnic groups and only one of these group’s plantations were infected. It had nothing to do with how they ran the plantations, only that the disease agent was issolated because the two groups never exchanged machettes.

He believes, but doesn’t quite have the research to prove, that many of the horribly virilent diseases that have emerged in hospitals over the last few decades can be explained, and then controled, by using these ideas. That these deseases have evolved so they can use the doctors and nurses as vectors and the patients as hosts. The key to pulling that off is to evolve to be benign in the vector and virilent in the host. Any difference between patient and caretaker is an opportunity waiting for a mutation to leverage. Newborns are particularly rich in these differences. So are patients taking antibiotics because they have suppressed their entire spectrum of bacteria. There is an ugly story about an outbreak of murderous diarrhea in Chicago. All cases were traced back to 27 hospitals; but how did they spread between the hospitals?

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