A new strain of coronavirus that has emerged in Britain has given rise to panic expectations: they say, the covid will become much more dangerous than before. Perhaps even the very "disease X" - a powerful pathogen that can lead to a pandemic with catastrophic consequences. For example, the collapse of the global economy. It is often said that another such "unexpected" disease will destroy all people. Or a sufficient number of them for the remnants of humanity to die out by themselves. Is it possible? If so, why hasn't humanity been destroyed throughout its long history? Let's look at it below.
There are many myths about infectious diseases. For example, it is believed that in the past people were inevitably destroyed by them, that only in our time it became possible to die from cancer or heart disease in the eighties. And before that, supposedly, microbes mowed down everyone without exception. Another misconception is that in the past, infectious diseases could not spread as quickly as they do now. After all, people lived at a great distance from each other, there was no transport capable of spreading microbes with the swiftness of the modern coronavirus. But today a truly dangerous disease can reach almost the entire population of the Earth in the shortest possible time.
Technically, this is not true, and sometimes it is not at all. And until we understand these myths, it will be difficult to understand why some epidemics claim many lives (up to every tenth on the planet), and others - hundreds of people, like the "SARS" of 2002-2003. It is also equally important whether diseases may appear in the future that threaten the very existence of our species.
How people began to get sick with infectious diseases
To understand how people in ancient times interacted with disease, it is enough to look at their African relatives today. Many of our traditional problems are taken from them, the monkeys of the Black Continent. Pubic lice are highly likely to have come to humans from gorillas millions of years ago, although the specific route of transmission is still being discussed by scientists. HIV was definitely caught by Africans from green monkeys in the 20th century (the method of transmission is just as controversial), and monkeys could play a significant role in the spread of Ebola.
However, it is epidemics among monkeys that are very rare. Green monkeys carry the simian variant of HIV (SIV) in themselves, but those infected with it live as long as those without. They have no symptoms (as, by the way, do some people). Chimpanzees have pneumonia, tuberculosis, and so on, but, as a rule, only age individuals with reduced immunity die from them.
Chimpanzees have analogs of human epidemics only if their species has recently received some kind of disease from another species. For example, in Tanzania, local chimpanzees often get sick with an analogue of our HIV, but, unlike green monkeys, they are not asymptomatic, but with real and negative consequences. Autopsies have shown that in the bodies of infected primates there is an extremely small number of immune cells (as in the dead human carriers), and the mortality rate among them is 10-15 times higher than among those chimpanzees that are not infected with this disease.
A similar picture is observed among those animals that are farther from humans than primates.So, in the European part of Russia a few years ago, many domestic pigs died from African swine fever, brought by migrant wild boars from the Caucasus Mountains, from the south. This disease, like Covid-19, is caused by a virus, not a bacterium, as in the case of the plague of people. In wild animals, especially in Africa, the virus is widespread, but almost all of its carriers there are asymptomatic: the pathogen lives in them in the position of a commensal, without causing harm to the owner, but also not benefiting. But when the Europeans tried to bring domestic pigs to Africa, it turned out that among them the virus is fatal in 100 percent of cases.
What is good for some, death for others
Where does this difference come from? The point is not only that any microbe normally cannot be an ideal killer of the species of its hosts, since in this case it will certainly die on its own: there will be no environment for its habitation. Another thing is also important: the host immune system quickly reacts to a pathogenic microbe and "learns" to either completely destroy it, or to keep the number of certain viruses or bacteria at a minimum level.
The typical result of this adaptability is the asymptomatic carrier, or "typhoid Mary". This is the name of a person whose body the infection does not cause any harm, but who at the same time remains a carrier of the pathogen. The asymptomatic carrier phenomenon was first discovered on Mary Mallon, an Irish cook who lived in the United States in the early 20th century. Her mother was sick with typhus during her pregnancy, and Mary's body "pressed down" the disease from the very beginning. As a result, her pathogenic bacteria could normally multiply only in the gallbladder.
When she worked in a particular house, people there subsequently fell ill with typhoid fever, at least five out of dozens of those infected with her died. Probably, there could have been fewer victims if she washed her hands, but, unfortunately, due to her moderate education, Mary said bluntly that she "did not understand the purpose of washing her hands."
Do not think that we are talking about a disease-exclusion. Different pathogens of cholera are carried by the same asymptomatic carriers, in the body of which they reproduce moderately, without leading to health problems. For some varieties of cholera pathogens, the ratio of "carriers" and "victims" is four to one, for others it is ten to one. Only a third of its untreated carriers die from syphilis (tertiary syphilis leads to death), others remain carriers. Tuberculosis develops into a dangerous, death-threatening form in only one case out of ten.
This situation is beneficial to pathogens. If they infected and killed every host, the number of man-hours that their carriers could spread the pathogen would be much less. Moreover, the microbes themselves do nothing for this: the host immune system is trying for them. Those who have it stronger, curb the pathogen and remain only carriers, and not sick in the literal sense of the word. Those with weaker immunity become victims of the disease. As a result, the number of descendants of persons whose immunity does not cope well with the disease falls, and the number of those with stronger immunity is doing its job, that is, it is growing.
This means that from a disease that has long been cohabiting with this or that human population, there can be no mass morale of people. But as soon as the disease gets to a place where they are not yet familiar with it, everything changes. An ideal case for infection is the delivery by travelers to new lands, where there were no such outbreaks before.
For example, in 1346, the Horde army was able to deliberately infect the Genoese garrison of Kafa (in the Crimea, now Feodosia) with a plague, throwing the corpse of one Tatar who died from it with a catapult into the fortress. Among the Tatars themselves, there were not so many who died from the plague: due to their long-standing contacts with the East, they acquired a certain resistance to the disease.
But in Europe and North Africa before this there was no plague for many hundreds of years, so the Genoese easily spread it across these regions.Historians estimate the total death toll at 70 million (more than in both world wars). In England, about half of the population died. Why is this, and not all one hundred percent, because Western Europeans did not have immunity to this infection?
The fact is that in a population normal in terms of genetic diversity, people - due to natural mutations - are not alike. For example, in the organisms of most Mongoloids, the ACE2 protein is presented more than in most Caucasians. It forms protein outgrowths on the surface of human cells, to which the SARS-CoV-2 virus, the causative agent of the current Covid-19 epidemic, clings. Therefore, as it was believed until recently, it is easier for it to spread in China, but it is more difficult outside countries with a Mongoloid population. Reality, however, has shown that proteins do not matter as much as a normal state apparatus. As a result, in fact, the Mongoloids suffered from the epidemic. But in another era, the situation could have turned quite differently.
It should be understood that there are many such subtle biochemical differences between people, so it is difficult to imagine a pathogen that could easily infect absolutely the entire population of the planet. Even in relation to those diseases that they have never encountered, some people can be very resistant.
For example, 0, 1-0, 3% of the Russian population is resistant to HIV due to the mutation of the CCR5 protein. The same mutation was once beneficial in countering the bubonic plague. That is, even if by some miracle HIV could spread by airborne droplets, it would not be able to kill all humanity infected with it: biochemical features would not allow it. Survivors would sooner or later return the population to a pre-epidemic level.
Perfect disease X
Often in the popular press they talk about the possibility of an accidental occurrence of an "ideal" disease, combining the high infectivity of measles (one sick person infects 15 healthy people), a long asymptomatic period of HIV and drug resistance, like in antibiotic-resistant bacteria. And even a small vulnerability to vaccines, like syphilis. Recall that it is difficult for him to create a vaccine, because antigens - compounds of a pathogen, "in response" to which antibodies are produced, are often found inside the cells of the pathogen, so the creation of antibodies that react to these "hidden" antigens is extremely difficult.
However, in practice, the occurrence of such a "super disease" is practically impossible. Nature does not have free breakfasts either for people or for the pathogens of their diseases. For its high resistance to drugs, vaccines and resistance to human immunity, the same HIV has paid with great specialization: it effectively affects only a small part of human cells and cannot get into it by airborne droplets. As a result, HIV affects less than fifty million worldwide.
Viruses that are well transmitted with the droplets that we breathe out cannot specialize only in immune cells, like HIV: they have to be "generalists of a wide range". And they cannot have sophisticated means of penetrating one specific type of human immune cells, like HIV. That is, diseases that are truly difficult to treat and recover, as a rule, are poorly airborne.
Diseases-exceptions can be well carried by air and destroy a significant part of the population, but the result will be that they will begin to act on natural selection among human hosts: those whose immunity fights better will more often survive, as a result, the virus will gradually cease to be dangerous for the population.
Often considered the most dangerous threat, antibiotic-resistant bacteria (for example, a number of staphylococci) also have serious limitations. Almost all of them today are conditionally pathogenic, that is, they are relatively safe for the body of a healthy person, since they cannot overcome its immunity.To be able to resist antibiotics, such bacteria change their parameters, become smaller in size and often show less ability to reproduce than competing species without strong antibiotic resistance. In other words, there are not very many candidates for "super disease". They, of course, can kill many aged and weakened people, especially in the form of nosocomial infections, but healthy citizens are too tough for them.
Some viruses try to bypass all these and some other problems due to great variability, constant mutations. The leaders in their frequency among the causative agents of common diseases are the influenza virus and, even more often, the mutating HIV. By constantly changing the composition of their outer shell, they escape the attacks of immune cells, but, again, at a great cost: the high mutation rate means that over time they lose some of their previous strengths. This is most likely one of the reasons why the HIV variant (SIV) in green monkeys does not cause noticeable harm to their health.
Last line of defense: numbers
Of course, all this does not mean that this or that disease, transmitted from individual to individual, cannot destroy the species as a whole. Undoubtedly, this is possible, but only with a combination of two factors: all individuals of the species live in a limited area, not separated by barriers, and their total number is not too large.
It is this disease that is now tormenting the Tasmanian devil - a predatory marsupial weighing up to 12 kilograms. These creatures have a difficult character, they hate each other. Even during the mating period, the male and female are constantly aggressive and bite each other. And three days after the onset of pregnancy, the female intensively attacks the male, forcing him to flee in order to save his life. Even 80% of her own cubs are corny mother-predator eats, leaving only four lucky ones alive.
In the 1990s, one of the individuals fell ill with a common cancerous tumor on the face, and this would not cause any problems in other species: the animal died - and that's it. But the Tasmanian devils are not like that: due to the habit of attacking the relatives of both sexes they meet, after a few years they re-infected with this tumor (through bites) about 70-80% of the entire population.
It is not clear whether the disease of these animals will be destroyed or not. Reducing their chances is the fact that Tasmanian devils have the lowest genetic diversity among all known predators and even all marsupials. The less variety, the lower the likelihood that someone will adapt to the disease due to the fact that his immunity is not quite the same as that of others. Australian authorities have created small "insurance" populations of these animals that are not infected with vector-borne cancer, and even if they become extinct in Tasmania, there is hope that the species will recover from these reserves.
In addition, recent work in Science casts doubt on the possibility of their extinction due to … the very fact of their decline. Cancer has caused such a drop in population density in populations of these animals that the disease is already spreading much more slowly than before. It seems that the likelihood of complete extinction of this species is low. However, taking into account his mores, very few people will be very happy about this.
But the example of the devils clearly shows that a person is well insured against mass extinction due to a new epidemic. We are not thousands, like these animals, but billions. Therefore, the genetic diversity of people is much greater, and an epidemic that is dangerous for some of us will not be able to kill everyone. We do not live on one not too large island, but are scattered across all continents. Consequently, quarantine measures can save some people (especially on the islands) even in conditions of complete death of populations in other places.
Let's summarize. The complete destruction of ours or some other common species due to an epidemic is a vanishingly unlikely event. Nevertheless, there is no reason to calm down.In 2018, the World Health Organization, in anticipation of such "super-diseases", introduced the concept of "disease X" (Disease X) - meaning a previously unknown disease that could cause a large-scale epidemic.
Less than two years after that, we are seeing Covid-19, a disease that is spreading like a pandemic and has already claimed many lives. It is difficult to reliably estimate the number of its victims, but for Russia this year the excess mortality rate during the epidemic is about 0.3 million. In the world, this figure is many times higher.
Of course, this is not a medieval black plague or smallpox. However, every lost life is important for humanity, therefore, tracking new "super diseases", as well as the creation of drugs and vaccines for them, is a matter that has to be dealt with by more than one generation of doctors and scientists.