*The information presented here has been taken from Jared Diamond's Guns, Germs & Steel.
I happened to be reading Jared Diamond's Guns, Germs and Steel just as COVID-19 was making its entry into Pakistan. And given the unfolding of events and all the conspiracy theories and confusion in people's minds, this book was extremely helpful in enlightening me to a few basic concepts of how microbes work, and how they have contributed to the evolution human societies.
Animals have always been the source of introducing new germs to humans
In his book Jared Diamond mentions that one of the reasons Europeans were able to conquer new worlds was Eurasia's significant abundance of options it had in large mammals for possible contenders of domestication. As people in Eurasia made their way through these possible contenders and settled on the species we see domesticated today, they were exposed to the germs these animals brought with them. As a result of that, human population in this part of the world developed immunity to these germs. Other far flung regions however, like Americas and Australia, were not so abundant with large mammals. And hence, since most of these regions never saw any domestication or very little of it, the populations in these areas were not as immune to these new germs. When Spanish conquistadors landed on South American shores, the germs they transmitted to local populations turned out to be much more lethal than the guns they brought.
In older times however, this transmission of germs was less frequent to other parts of the world than it is today, simply because of the ease and frequence of international travel. By a rough estimate for example, by the time Wuhan was locked down, millions of people had already left the city. And so unlike Spanish conquistadors who brought their germs to South America years after their own population was immune to it, today we see the simultaneous emergence of viruses with no clear signs of herd immunity in sight.
Microbes mutate as they jump from one host species to another
Since viruses can be simply described as a package of genetic information (RNA or DNA), it can be clearly deduced from basic evolutionary theory that their main goal is to survive and multiply. And to survive, every gene must adapt to its host environment through random mutations. Same is the case with viruses. Since their main goal is to multiply and survive, as they jump from one host to another, mutations occur which make them more resilient and suitable to their new host's environment. This is one of the basic reasons we see mutations in any virus as it spreads more.
There are various strategies microbes adopt to maximize their rate of spread
One big misconception about microbes is that they're out to kill their hosts. This however, would be bad for microbes themselves if their host dies too soon since they would not have had the time to move from one host to another. So microbes instead try to increase the time for which they can remain in their host and maximize their transmission to multiple carriers. A clear example of this would be COVID-19's longer incubation periods and asymptomatic carriers.
Depending on the genetic structure of a microbe, the modes of transmission can significantly vary. Some of these strategies are outlined below.
- Passively passing from one host to another. One example of this could be consumption of one host's meat (animals) by another host (humans).
- Germs that hitchhike from one infected host to another. This can be done through flies and insects. AIDS too falls in this category since it is transmitted through sex.
- By influencing the anatomy of host as in Smallpox.
- By influencing the behavior of victim like flu and COVID-19. This is the most aggressive strategy of transmission since it causes physiological changes in host that makes it actively spread the disease.
This post is mostly a paraphrasing of Jared Diamond's text, as mentioned in the beginning. To anyone interested in the wider topic of how societies evolved through time, I'd highly recommend reading Guns, Germs and Steel.