TheAzn said:
Why did these plagues come in waves (Justinian Plague, Black Death of 1347, etc.)? I mean, even when the Silk Road is restricted, the contact and trade never stopped. Rats will always get to be transferred from one end of Eurasia to the other.
Also, I have heard that all of the main Bubonic plagues began in Yunnan, China.
Do you guys know of any very credible books that deal with the origin and "Birthday" of the Bubonic plagues?
Thank you
Evolution by punctuated equilibrium explains why epidemics usually come in waves.
Hypothesize that there is a species of pathogen with many varieties. It attacks a species of host which itself has many individuals. Each variety can overcome a certain type of antibody or defense of the host. Hosts have a wide variety of antibodies and other defenses, some of which are hereditary and some are developed by the individual in response to sickness. For example, every host of vertebrate can have a histoimmune profile consisting of the specific antibodies it makes to resist pathogens.
The general pathogen infects some hosts all the time. For long periods of time, the pathogen is in equilibrium with the host. The pathogen isn't spreading, infects only a fraction of the host, and causes very few symptoms. However, there is a finite rate at which mutants and other inherited variations occur in the pathogen. Most mutations end up killing the pathogen that has it. For instance, the pathogen could become more sensitive to the antibodies in most hosts. However, a few mutations improve the ability of the pathogen to reproduce and survive. A few mutations will be immune to the antibodies or host.
A new variety of pathogen, that can overcome all the available antibodies in the host, can develop in a single mutation or in a series of mutations. The series of mutations are especially likely if there are small isolated communities inside the larger population of hosts. I won't go into into the second possibility, but it has been studied a great deal in both epidemiology and paleontology. The new variety in the isolated community inevitably gets out to infect the larger host population. Either way, the general host population meets a new variety of pathogen that can overcome the defenses left from the last outbreak.
The pathogen is no longer at equilibrium with the host. The new variety of pathogen spreads rapidly. The pathogen spreads until it has either killed all the individuals that don't resist it or when the majority of individuals have developed antibodies to resist the pathogen. Then, this variety of the pathogen either goes extinct.
Once the new variety of pathogen becomes extinct, the extant varieties of pathogen are in equilibrium with the host. As many hosts catch the disease as become immune to it every day. Because there are so few each day who catch the disease associated with the pathogen, the pandemic becomes unnoticed. The cycle can start over.
New variety develops by one or a few sequential mutations. Overwhelms most of the host. Maybe it kills or immunizes the sensitive host. It leaves a few hardy individuals immune to the new variety. Host population goes into equilibrium with the large population of hosts.
At some point, there will be a mutation or other variation that overcomes the defenses of the remaining host. The newest variety is different from the previous variety in that it can overcome the antibodies of the hosts that are left. Then a new epidemic breaks out.
Basically, it is the same reason that new species of organisms suddenly appear in the fossil record even though evolution is slow. After a mass extinction, there are isolated communities of the organisms. Mutations have less competition than before. New species develop rapidly, taking over host environments. However, eventually the system goes back to equilibrium until the next mass extinction.
This seems antiintuitive to some people. Here is how I think of it.
The development of new varieties is slow, just as Darwin described. However, Darwin was wrong about one thing. The extinction of a species can be very fast. Therefore, the time that a species exists at equilibrium can be very large. Sudden changes of any type can make a new variety that spreads like a wildfire. Like a wildfire, it burns itself out. It immunizes the host, or it kills all susceptible hosts. When a rabid variety goes extinct, there is an illusion that every last member of the species has disappeared. This is wrong. A new variety of that species can reappear at any time. However,
Let's us consider bubonic plague, which is caused by the Yersinai pestis bacterium. Even though it is considered one species, the individual bacterium varies quite a lot. Different individual bacterium have differing ability to infect people with different antibodies. Like all gram-negative bacteria, it conjugates. It exchanges genetic material with others of its kind. It reproduces by splitting, but it also can trade genetic material with others. So there are communities of Yersina pestis bacteria that are similar genetically, but vary a bit from other communities. Occasionally, a Y. pestis bacterium mutates. Most mutations die quickly.
Yersinia is never wiped out. Not only are there many communities in human hosts, but there are many communities in rodent hosts. Every species of rodent has some Yersina, and there are small populations of humans that have it. Most varieties of Yersinia are hardly noticed unless they can overcome the majority of antibodies.
Occasionally, a new variety develops which can overcome all the antibodies among the hosts. It spreads like wild fire. Eventually, people recognize it. They quarantine the victims. They give specific antibiotics to cure the victims. The new variety goes extinct or evolves into a less active bacteria.
A mutation occurs that is immune to common antibodies and to the antibiotics. It spreads. It makes people sick. The cycle occurs again.
Note that today, scientists can identify the new strains. They know these germs are constantly changing. One can never catch the same cold twice. This is why many health organizations try to stop the new varieties soon after they develop.
When I was younger, I read "The Coming Plague" by Laurie Garrett. Some of this information is outdated now. She takes an alarmist stance which in my opinion is slightly exaggerated. However, it describes some of the details very well.
I also recommend: "The Evolution Explosion" by Stephen R. Palumbi. It describes the situation with some pathogens, but not in detail. He is not as alarmist as Palumbi, so maybe you will find what he says less questionable. He makes a strong connection between Darwin's theory of evolution and the spread of disease. I think he is a clearer writer than Garrett, but you can decide. Palumbi focuses on the general theory, and doesn't go into the very fine details.
There are a bunch of more recent books that describe the situation better. I have been out of the loop for some time.
My own opinion is that what we see today is a sort of break. The cycle of disease has been going on for millions of years. Things aren't going to Hell. Things are going back to normal.
