Salvador Macip - Modern Epidemics

The other important factor when defining the aggressiveness of an outbreak is the severity of the symptoms it causes. These can range from a slight fever and feeling out of sorts (as with the common cold) to death. It’s said that the virulence of the infection is determined by the intensity of the effects it has in people. An infection that spreads quickly (one with a high R0) usually has low virulence, but, even so, it can still constitute a major health problem, as we’ve seen with COVID-19. Then again, if a disease kills a high percentage of infected people, the ease of contagion tends to be much lower, so it’s unlikely to cause a pandemic (as it will remain localized). A typical example of this would be Ebola, which has very high lethality, but it rarely goes beyond an outbreak or, at most, an epidemic. A combination of easy transmission and high virulence is what is most dangerous. Fortunately, this combination is highly improbable.

Animals are sometimes part of infectious cycles in which they become reservoirs – that is to say, a place where microbes can accumulate and from which they can infect humans in future. Very often, the animals that act as reservoirs aren’t affected by the presence of the microbes and show no symptoms of disease either. The existence of reservoirs makes it very difficult to eliminate microbes completely. Examples include pigs and birds (common reservoirs of influenza viruses), and mosquitoes (reservoirs of malaria). Many of the major recent pandemics come from viruses that have jumped to humans from their reservoir animals, for example monkeys in the case of AIDS and, probably, bats in that of COVID-19.

To conclude this initial chapter, I will give a brief account of the three most important kinds of microorganisms from the medical point of view: bacteria, viruses and fungi.

Bacteria are microbes consisting of a single cell. After the sixteenth century, there were theories postulating that diseases were transmitted by a kind of ‘seed’ that went from one person to another but, without the necessary instruments, it was impossible to confirm this idea. It wasn’t until the seventeenth century, when the Dutchman Antonie van Leeuwenhoek invented the microscope, that it was possible to discover these ‘germs’, as they were originally called. In one of his first observations, he described something like abundant ‘very little animalcules’, which were everywhere. He named them animalculae and proposed that they were responsible for infections. It was only possible to demonstrate this in the nineteenth century and, in 1838, Leeuwenhoek’s animalculae were officially named bacteria.

There are many classes of bacteria and they can have very different forms. The most typical are round and they’re called cocci , while the elongated ones are known as bacilli . They are found everywhere, and in abundance. For example, there are 40 million bacteria in every gram of earth, and 1 million for each millilitre of water. If we counted all the bacteria on the planet, we would get a figure with thirty zeros. It’s therefore believed that most of the types that exist haven’t yet been discovered or identified.

As I said in the beginning, bacteria participate in many important processes in our ecosystems, for example recycling nutrients through nitrogen fixation and putrefaction. They are also necessary for fermentation: without the work of bacteria, there would be no cheese, wine, vinegar or yoghurt. Scientific advances have made it possible for us to carry out research with them in laboratories, and new bioengineering techniques allow us to use them to produce insulin and antibodies.

Bacteria multiply by means of a process called binary fission , during which a bacterium divides into two identical parts. The genetic information of a bacterium is contained in a single circular chromosome (recall that humans have twentythree pairs of X-shaped chromosomes). However, bacteria can also have isolated genes, independent of the chromosome, which they frequently obtain through exchanges with other bacteria. These ‘extra’ genes are called plasmids and they are very important in infections. Plasmids allow bacteria to acquire new capabilities, for example resistance to an antibiotic, or generating a lethal toxin, as happens in the cases of diphtheria and cholera. Other diseases caused by bacteria are tuberculosis and plague.

The first signs of the existence of microorganisms smaller than bacteria date back to the 1870s when some Dutch scientists realized that there were mysterious agents that could pass through the filters that held back bacteria and, having done so, cause infections. The first virus was described in 1898 and, since then, more than 5,000 different types have been identified. As with bacteria, it’s believed that most of them haven’t been discovered yet.

Viruses are the tiniest life forms in existence (between 100 and 500 times smaller than bacteria), although many people debate whether they are really alive or not, the reason being that they are not able to function alone because they must invade a cell in order to divide. In fact, viruses are nothing more than a group of genes surrounded by a more or less complex capsule that enables them to penetrate the cells of animals, plants or even bacteria themselves. Unlike the latter, they tend not to bring any benefit to the organisms they infect: they are more like parasites.

They are also the planet’s most abundant organism and are found in all ecosystems. If we lined up all the viruses in the oceans, for example, they would extend 100 times further than the limits of our galaxy. Some are innocuous for humans and others can cause chronic (like hepatitis) or acute (like influenza or the common cold) diseases. Viral infections tend to be generalized and don’t cause pain, unlike the bacterial kind, which are normally localized and cause inflammation or pain in the affected area.

However, viruses can be used to our advantage. They are essential, for instance, as tools in a great number of laboratory experiments. They enable us to introduce genes into cells, which can be useful not only in experiments but also for treatments like gene therapy. And, for quite some time now, viruses that attack bacteria (called bacteriophages ) have been studied with a view to their use as an alternative to antibiotics.

Our current knowledge of genetics and molecular biology allows us to manipulate viruses in every conceivable way. In the opinion of the Galician virologist Luis Martínez-Sobrido, professor and head of a virology research group at the Texas Biomedical Research Institute in San Antonio (USA), ‘there’s no end to the possibilities’. He adds: ‘In the last sixty years since Watson and Crick discovered the structure of DNA, more progress has been made in the study of living beings than in the entire history of humanity.’ 1

Certainly, for some years now we’ve been able to play with the genes contained in a virus, putting them in or removing them as we see fit, depending on whether the aim is to eliminate one of their functions or add a new one. One example is the use, in studies for the Ebola vaccine, of VSV, a virus that’s inoffensive for humans, into which Ebola genes have been introduced in the hope that this will cause an immune response but without developing the disease. ‘From the technical point of view, it’s only a matter of time before we’ll be able to do anything we want with viruses’, says Dr Martínez-Sobrido. ‘As they said in the film, I Am Legend , viruses are like cars. With a good driver at the wheel, a car will take us where we want to go, and we’ll benefit from that. A bad driver, however, can cause death.’