Modern Epidemics. Salvador Macip
not an outbreak turns into an epidemic, a pandemic or becomes endemic depends on many factors, among them the speed at which it spreads and the virulence of the disease it causes.
Which disease spreads fastest?
It has been estimated that the 1918 flu pandemic had an R0 of around 4 (meaning that each affected person infected four more). Seasonal flu (the annual one) normally has an R0 of between 1.3 and 3. The A(H1N1) flu pandemic of spring 2009 had an R0 of only 1.4, while that for measles is 15, which means that it’s a much more contagious disease than the others. The R0 for smallpox was between 5 and 10, and for AIDS it’s between 10 and 12. That for COVID-19 is still being calculated, but it could be close to 2 at the very most.
An infection’s ability to spread is defined by a variable called R0, which is to say the number of new infections that, on average, each person with the disease can cause or, in other words, how many more people can catch the disease from each already infected person (see box). It’s also important to know the amount of time over which a person can infect others. With most infectious diseases there’s a period of incubation when, although the symptoms haven’t yet appeared, the microbe can often be transmitted. The longer the period, the greater the risk of the outbreak spreading because the infected person normally doesn’t know and appropriate measures to avoid contagion aren’t taken. One known example of this is COVID-19, which, going unnoticed in the early (between ten and fifteen) days, can be contagious. The extreme case is AIDS, which may not show any signs for years. In some cases, infected people will never develop the disease but can, nevertheless, pass it on to others. They’re called carriers.
Typhoid Mary
Mary Mallon (1869–1938) has gone down in history as ‘Typhoid Mary’, the first person to be identified in the United States as a carrier of typhoid fever (a disease caused by the bacteria Salmonella, which is transmitted through contaminated food or drink) without ever being ill herself. Mary was a kind of epidemic on two legs. She infected fifty-three people in her lifetime but always denied that she was to blame. Moreover, she never wanted to leave her job as a cook, despite the very high chances that she would infect people through the food she prepared. When she was banned from cooking, she even changed her name so she could keep doing her job and thus continue to infect and kill her clients.
Mary, who worked in New York, was finally forced to go into quarantine. When she died (of pneumonia) she was still in isolation. It’s believed that she could have been born with the infection, as her mother had the disease when she was pregnant.
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.
Bacteria
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.
Viruses: the smallest life form?
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