2 2. OC43 beta coronavirus is also the cause of common cold, and just like the 229E, it infects the upper respiratory tract and affects the same organs. The symptoms are also the same as well as the recovery time. Subjects with other health problems can develop pneumonia too [2].
3 3. NL63 alpha coronavirus causes many common symptoms and diseases; it infects the upper and lower tract and can cause croup and bronchiolitis [3].
4 4. MERS-CoV, the beta coronavirus, is the cause of Middle East Respiratory Syndrome or MERS; the symptoms are coughing, shortness of breath, and fever. Sometimes, the subject can also present pneumonia and gastrointestinal symptoms, such as diarrhoea. Some laboratory-confirmed cases of MERS-CoV are reported asymptomatic, i.e., the subjects do not present symptoms. Most of these cases are detected after a thorough tracing of laboratory-confirmed cases [4].
5 5. HKU1 beta coronavirus causes upper respiratory diseases with common cold symptoms and the subject can develop pneumonia and bronchiolitis [5].
6 6. SARS-CoV, the beta coronavirus, causes severe acute respiratory syndrome (SARS), it starts with a high fever, more than 38°C, the subject presents chills, headache, and muscle pain; some cases have mild respiratory symptoms, and some patients have diarrhoea.
7 7. Within a week, the subject presents a dry cough or dyspnoea that can progress to hypoxemia. Some cases even require intubation and mechanical ventilation. The white blood cell count decreases and many people have low platelet counts [6].
8 8. SARS-CoV-2, the novel coronavirus which is the cause of COVID-19, affects people in different ways. In most cases a moderate illness is present and the subject recovers without hospitalization. The symptoms are fever, tiredness, dry coughing, aches and pains, diarrhoea, headaches, conjunctivitis, rash on the skin, and loss of taste or smell. There could also be difficulty breathing, chest pain, and loss of speech or movement [7].
3.3 The SARS-CoV-2 Pandemic Impact
The SARS-CoV-2 and the COVID-19 disease impact is a multifactorial phenomenon to measure it we have to take into account some factors (Section 3.1), so let us only take two of them for the moment; its dissemination capacity and its capacity to kill a human being.
The propagation of viruses has to do with the possibility of passing to any organism and the SARS-CoV-2 virus gets this feature from the Coronaviridae family (Section 3.3.2), which can be considered very high; on the other hand, the lethality of the SARS-CoV-2 is no higher than 2% (Section 3.1), which is low.
However, both factors together create a combination that strongly impacts humans, as their propagation and lethality take on a large number of lives, given the permanence of the virus in human beings.
In this sense, the impact is very high since this coronavirus has a very similar spread to any flu; however, its lethality is very high compared to this disease, as up to date there is no effective treatment or vaccine available; unlike the A H1N1 influenza pandemic in 2009 that had an effective treatment and the vaccine was available few months later.
In this regard, any alert or official warning that the population is facing an epidemic process or a pandemic, in general, has to do with a minimum number of people infected that have had a microbiological corroboration.
In the case of an unknown illness, this verification can take days or even weeks; this is a limitation that defers the notification. So, once this alert has been issued, the authorities should state it is a pandemic and not an epidemic event, according to the definition that at least two continents present infected cases, regardless of the number of them.
From the epidemiological point of view, there should be an algorithm that not only considers the number of confirmed cases, as continuous improvement of the means of transport makes contagion and dissemination of illness much faster nowadays.
3.3.1 RNA Virus vs DNA Virus
There are two main groups in the classification of viruses and they are related to the genetic material a virus uses to replicate, whether it is in a DNA or an RNA host.
DNA viruses are formed by the families (Source: [8]; change family by the corresponding family name): Adenoviridae, Papillomaviridae, Parvoviridae, Herpesviridae, Poxviridae, and Anelloviridae.
While RNA viruses are formed by the families (Source: [8]; change family by the corresponding family name): Reoviridae, Picornaviridae, Caliciviridae, Arenaviridae, Rhabdoviridae, Coronaviridae, and Herpesviridae.
Given the purpose of this chapter, the reader is invited to consult the source cited in each of the classifications, if he/she wants to delve into this topic.
3.3.2 The Coronaviridae Family
Coronaviruses are transmitted by respiratory secretion sprays, with different incubation periods estimated between 2 days and a week, after these 2 weeks, the virus is eliminated from the body. The virus replicates in the host cell, passing to the Golgi apparatus until they are finally released by exocytosis. In general, coronaviruses induce 15% of colds and flu, and their effect intensifies in winter and early spring.
Coronaviruses infect a large number of birds and mammals, particularly the SARS CoV-2-index-SARS-CoV-2; they are responsible for severe respiratory diseases, which can be spread even to babies; and in few cases, they are responsible for neurologic syndromes.
The virus remains in the upper respiratory tract and exhibits minimal immune response. Due to its mutation capacity, it persists in the invaded species.
3.3.3 The SARS-CoV-2 Structural Proteins
Four structural proteins form the SARS-CoV-2, the Spike, Membrane, Envelope, and Nucleocapsid proteins; there are other groups related to SARS-CoV-2 like the non-structural proteins, and SARS-CoV-2 putative accessory factors; although, there are also protein groups with important structural differences related to it, such as those of the MERS, and SARS-CoV proteins.
3.3.4 Protein Representations
Proteins are formed by amino acids; they are the fundamental units of every living organism; they are transformed into tissues, muscles, skin, or nails, but they can also be converted into accelerators or retardants of chemical or physiological processes. Life could not be understood without them.
The first representation of a protein is a succession of amino acids; it is like placing one amino acid after another, this representation is called linear representation or sequence. Then, three representations take place in the three-dimensional space that are related to the form the amino acids settle.
When protein amino acids cluster, they take forms like alpha helices and beta sheets, this is called a secondary representation. When these small structures bind together, the protein has a tertiary representation. Finally, if the protein is so large that it is made up of two or more tertiary structures, then it is said to be a protein with a quaternary structure.
A protein may be made up of few amino acids or thousands of them; however, their number has nothing to do with the size it adopts but with its regulation. Suffice it to say that as an example the SARS-CoV-2 structural protein (Spike) has 1283aa and yet it only has few microns.
Note 3.1 Here, it is important to mention that viruses are formed by proteins, and although at the time being, it is still discussed whether they are living organisms or not, they are also formed by proteins.
3.4