Understanding the spread of infectious diseases in one article

In the course of human evolution, infectious diseases always pop up from time to time to show their presence, such as the plague that occurred in China from the Western Han Dynasty to the Eastern Han Dynasty (approximately the 1st to 3rd century AD), which appeared so unpredictably; and the plague (Black Death) that broke out in Europe in the 14th century, claiming the lives of millions of people.

Historically, these infectious diseases of varying degrees have appeared all over the world, and today, ancient infectious diseases still exist (except smallpox, which has been eradicated). Of course, new infectious diseases continue to emerge, such as the new coronavirus outbreak in late 2019. This "emerging" infectious disease refers to an infection that has newly appeared in the population or already existed but the incidence rate has suddenly increased rapidly within a certain geographical range .

The emergence of infectious diseases

Infectious diseases are caused primarily by pathogens, which can be bacteria (such as tuberculosis caused by Mycobacterium tuberculosis), viruses (such as the common flu), fungi (such as various forms of ringworm caused by fungi), and parasites (such as malaria, which is caused by tiny parasites transmitted by mosquito bites).

So why do these pathogens cause disease?

If we quote the view of British evolutionary theorist Richard Dawkins, author of "The Selfish Gene": people are born selfish, or to be more intuitive - genes are born selfish.

So the mutations or gene mutations of bacteria and viruses have nothing to do with you. Their changes are only for themselves . If their changes affect you, such as causing a disease.

Sorry, no, they won't apologize at all. And they don't consider the impact on you at all, but once this impact threatens their own (interests), the change will stop.

Because if a pathogen causes a disease that is too severe, the people who get sick will die quickly, and the pathogen will disappear from the earth.

So they want to avoid such incidents. Only by giving patients a chance to breathe and passing the pathogens to the next host can their genes be passed on.

Take rabbits as an example. In the 19th century, wild rabbits were a disaster in Australia. The Australian government tried many methods but failed to reduce the number of rabbits. Finally, they had to introduce myxomatosis (a virus targeting wild rabbits with a very high mortality rate) from Brazil to Australia. In the first year of introduction, the effect was very good, and the mortality rate of wild rabbits reached 98%. It seemed that the wild rabbits would not live long. However, in the second year, the myxomatosis virus was also used, and the mortality rate of wild rabbits was greatly reduced, and finally stabilized at about 25%.

Why has the virus’s mortality rate decreased?

Because the virus realized that if all the rabbits died, they would not be able to find a host and would naturally go extinct. So before they could evolve the ability to infect other species, they had to make the rabbits die more slowly.

How does human behavior affect infectious diseases?

Back to the pathogens themselves, they generally only have the ability to cause disease and their natural transmissibility is low, so at this time, human behavior begins to play a role.

Most of the initial infectious diseases originated from zoonoses . In ancient times, agricultural development was one of the most common ways for humans to change the environment. In order to develop better, humans often went to unknown places to open up new fertile land, and then inevitably came into contact with host animals and were infected with hitherto unknown diseases but usually already existed in animals.

For example, the Hantavirus that causes hemorrhagic fever in South Korea. The virus was originally naturally transmitted by field mice, Apodemus, which reproduced and lived in the fields for generations. When humans cultivated the fields, they came into contact with infected Apodemus and became a new host of the Hantavirus. This rodent-borne Hantavirus infection has existed in Asia for hundreds of years, and there are nearly 100,000 cases of infection with the virus in China every year.

The Junin virus, which causes Argentine hemorrhagic fever, has a similar history to the Hantavirus. The natural host of this virus is rodents. Since humans have converted most grasslands into corn plantations, which is more conducive to the survival of rodents, human cases have gradually increased with the expansion of corn agriculture.

On the other hand, with the migration of human beings and the outbreak of wars, especially the great geographical discoveries during the "Age of Exploration", the exchanges of culture, economy, beliefs and even diseases among various regions of the world were promoted.

The pathogens that cause diseases originally stayed in their own territories. In the thousands of years of evolution in nature, a delicate balance has been maintained between the immune system of organisms and invading pathogens. However, the means of transportation created by humans brought them to a place they had never been to before , allowing them to "kill everywhere" among people who lacked immunity to them.

The most famous example is the spread of AIDS. Although the exact ancestor of HIV-1 is uncertain, the first human infection may have originated from a man in rural Kinshasa, Congo, Africa. The man's virus strain was similar to the virus strain in the sooty black-browed macaque (a small to medium-sized monkey living in Africa, which is recognized as the source of HIV-2).

Although infections have occurred, as long as the infected people are isolated, the virus will not "run out". However, with the increasing number of people moving from rural areas to cities, such isolation is basically rare.

And the urbanization of rural areas has allowed infections in more remote rural areas (which may have once been unknown) to spread to a larger population.

Once in cities, “emerging” infections have more opportunities to spread among local populations and even further afield along highways and inter-city transport routes and by air travel , a step that was critical for HIV and fueled today’s global pandemic.

The origins of the HIV pandemic

In addition, human behaviors such as unhygienic sex or intravenous drug use are also common ways to promote HIV infection . Therefore, properly regulating individual human behaviors is of great significance in controlling "emerging" infections.

Ironically, as the work on AIDS prevention demonstrates, human behavior remains one of the weakest links in our current scientific knowledge.

But this does not mean that human behavior is completely wrong, because it is impossible for us to avoid contact with animals, it is impossible for us to stay away from home forever, and it is even more impossible for us to avoid contact with others forever.

Because of this, when infectious diseases are transmitted from animals to humans, human behavior will intervene strongly to reduce their devastation of life.

In fact, what we need to do is not to completely eliminate infectious diseases. In fact, this is also very difficult. Even if we have developed a vaccine to deal with it, these pathogens will have one skill - mutation , which is completely a bug...

What we really need to do is to constantly find ways to coexist harmoniously with them during the evolution process, so that they become less deadly.