Deadly influenza

The Spanish flu occupies a preponderant place among pandemics and it is considered one of the worst, between 1918 and 1920 it destroyed part of humanity. It is estimated that around 100 million people died from this flu (from 3 to 6 % of that time world's population). Even the total number of civilian and military deaths during First World War was lower than the casualties generated by this pathology (^{Luthy, Ritacco & Kantor, 2018}; ^{Pané, 2020}).

For the record, the term Spanish flu arises because Spain was one of the countries with the highest number of cases, with approximately 8 million people infected and 300,000 casualties (^{González García, 2013}). But the nickname is due to a political situation. The United States and France, with numerous cases as well, were involved in the First World War and with the political, social and economic upheaval, the epidemic was kept a secret. Spain was a neutral country and due to its freedom of press, the pandemic was reported (^{World Health Organization (WHO), 2020}).

The American soldiers who landed in Europe in April 1918 were already carrying the virus. One of its peculiarities was that it mainly affected people who were between 20 and 40 years old, which was very different from other pathologies in which older people are the risk population. At that time, there was no vaccine or antibiotic treatment to treat secondary infections associated to the virus, and efforts were limited to quarantine, disinfection and hygiene, and crowd avoidance (^{Pané, 2020}).

The first influenza A pandemic was the so-called Russian flu and the second one, the Asian flu. The Russian flu developed between 1889 and 1890 with a very high morbidity rate: approximately 40 % of the world's population fell ill. It caused one million deaths, especially people over 65 years old and young children, due to complications. The influenza A virus (subtype H2N2) was found in birds and it is claimed to have first emerged in Russia (Rodriguez-Maffiotte, 2018) and spread for three years until 1892.

Subsequently, the Asian flu, which emerged in 1957-1958, caused around 1,100,000 deaths. This pandemic especially affected children, adolescents and young adults. Despite the fact that the lethality of the Asian flu was very low, its rapid expansion had a negative impact on the economy and, together with the 1918 epidemic, it recorded the highest number of deaths in the 20th century (^{Acuña, 2004}; ^{Castañeda Guillot & Ramos Serpa, 2020}).

Some authors believe that the Asian flu originated from a mutation in wild ducks that combined itself with a pre-existing human strain. The virus was first identified in Guizhou, China, and spread to Singapore, from there to Hong Kong and then to the United States. The rapid development of a vaccine against the H2N2 virus and the availability of antibiotics to battle secondary infections limited its spread, and the epidemic's mortality was kept under control. After 10 years of evolution, the Asian influenza virus disappeared but gave rise to a subtype that eventually caused a new pandemic: the Hong Kong flu (^{Castañeda Guillot & Ramos Serpa, 2020}).

The so-called Hong Kong flu developed between 1968 and 1970 and affected almost one million people. The virus responsible for this pandemic is still circulating today and it is considered one of the seasonal influenza strains (Rodriguez-Maffiotte, 2018; ^{Tuells, Caballero, Nolasco & Montagud, 2012}). The first record of the outbreak in Hong Kong appeared on July 13, 1968, and by the end of the month it had reached Vietnam and Singapore. In a few months, carried by soldiers returning home from the Vietnam War, it reached the Panama Canal Zone and the United States.

Swine flu was the second pandemic caused by the H1N1 flu virus and it was first identified in the United States in April 2009 and spread rapidly around the world. Mexico was one of the first countries to detect cases, according to the United States Centers for Disease Control and Prevention (CDC; ^{Withington, 2009}). It is a new strain of H1N1, which originated when avian, swine and human influenza viruses combined themselves with a Eurasian swine flu virus -that is why it is known as swine flu. The clinical picture is similar to that typical of seasonal influenza, of spontaneous cure, although the clinical spectrum is extensive, ranging from asymptomatic cases to severe or fatal pneumonia (^{Rafart, Cuesta & Agulló, 2009}). Compared to its high expansion (it is estimated that between 11 and 21 % of the population was infected), mortality was not so high. Influenza A is estimated to have caused more than 400,000 deaths. At least one in five people in the world was infected during the first year of the pandemic (2009-2010) and the mortality rate was only 0.02 % (^{Salinas, 2009}). Finally, after 14 months of pandemic, the WHO declared its end on August 10, 2010.

The reign of corona viruses

Coronaviruses (from the Latin corona (crown) or a halo appearance) are named after the way they are visualized under the electron microscope. They are a family of viruses discovered in the 1960s, of unknown origin. Regarding symptoms, the different types generate different diseases, from a cold to a severe form of pneumonia, and almost all of them are treatable, according to the ^{Consejo General de Colegios Farmacéuticos (2020)}. In that sense, they are not life-threatening; moreover, most people will eventually catch a coronavirus, the simplest one, for instance: a cold. Infection is most common in autumn or winter, time of year for angina, flu, allergies and various respiratory infections.

In the 1960s they were first described in the nasal cavities of patients with colds, and only 6 species of coronaviruses that could infect humans (HCoV) and cause respiratory disease were known. These were: HCoV-229E, HCoV-OC43, HCoV-NL63, HKU1, SARS-CoV and MERS CoV, which generally cause mild upper respiratory tract infections (^{Matoba et al., 2015}; ^{Sipulwa, Ongus, Coldren & Bulimo, 2016}; ^{Zhang et al., 2018}). They can rarely generate severe infections in pediatric population and elderly adults. They are endemic globally and account for 10-30 % of upper respiratory tract infections in adults.

SARS-CoV started in November 2002 in Guangdong province (China) and affected approximately 8,096 cases in 29 countries, causing 774 deaths. Until then, coronavirus infections in humans only caused mild infections in immunocompetent patients. The epidemic could be controlled in a short time with few fatalities, without reaching a serious contagion multiplication (^{Sampathkumar, Temesgen, Smith & Thompson, 2003}). Defined as a severe form of pneumonia, its symptoms consist of fever above 38 degrees, respiratory distress and other symptoms such as chills, myalgia, headache and malaise (^{Castro-Sansores & Góngora-Biachi, 2003}). The mortality of SARS-CoV has been estimated at approximately 10 % (^{Sampathkumar et al., 2003}). Since 2004, no cases of SARS-CoV have been reported.

Ten years later, in 2012, another highly pathogenic coronavirus emerged and it was first identified in Saudi Arabia: the Middle East respiratory syndrome coronavirus (MERS-CoV). The first case was a 60-year-old Saudi Arabian man suffering from acute pneumonia who died of kidney failure. The virus dynamics evolved and the first cases were reported in England (^{Valentin, Montero & Florentini, 2020}), Germany and in several Middle East countries (^{Bratanich, 2015}).

From October 2019 onwards, more than 2,400 cases have been reported in different countries, with a fatality rate of 35 %. The symptomatology included severe respiratory problems, in addition to fever, cough and shortness of breath -although at first it may be asymptomatic (^{Castro-Sansores & Góngora-Biachi, 2003}). In the most severe cases, vomiting, diarrhea and even bloody expectoration also occur. Since the emergence of SARS, a large number of coronaviruses have been discovered in bats, which serve as a host. Camels are likely to be an important reservoir for this type of coronavirus and, according to ^{WHO (2020}; ^{ECDC, 2020}), an animal source of infection in humans.

The current corona: COVID-19

The story begins in Hubei province in Wuhan (China), in December 2019, with a group of patients diagnosed with pneumonia of unknown origin. Most of them were epidemiologically linked to a wholesale market of live and unprocessed fish, seafood and animals in Hubei province. A high consumption of animal protein, including exotic animals such as snakes and bats, together with poor hygiene standards in food markets made zoonosis possible: the transmission of viruses among animals and from animals to humans (^{Ahmad et al, 2020}; ^{Mackenzie & Smith, 2020}).

In mid-December 2019, the first hospitalized cases were reported with symptoms of acute respiratory distress. Most of the patients claimed to be directly or indirectly related to the above mentioned market, which was closed in January 2020. Days later, Chinese researchers announced that they had identified a new type of coronavirus (new coronavirus, 2019-nCoV), ruling out other pathogens such as SARS-CoV, MERS-CoV, influenza virus, avian influenza and adenovirus. At that time, the Chinese crisis (due to the contagion levels) began to spread the news of what was happening and the cases kept on rising (^{Ramos, 2020}), which made it more difficult to track. The disease was named COVID-19, initially novel coronavirus (2019 nCoV), and was mistakenly thought not to be highly contagious as there was no record of person-to-person infection.

After 10 days, there were numerous cases reported (571) in 25 provinces in China. Thereafter, the number of infected patients increased exponentially in mainland China and in different countries, including Vietnam, Taiwan, Thailand, Sri Lanka, Cambodia, Japan, Malaysia, Singapore, Republic of Korea, Nepal, United Arab Emirates, Philippines, India, Iran, Canada, United States, Finland, France, Spain, Australia and Germany (^{Pulcha Ugarte, Pizarro-Lau, Gastelo-Acosta & Maguiña-Vargas, 2020}).

The first case reported in the American continent occurred on January 19, 2020 in the state of Washington (United States). Also, on January 24, the first case of COVID-19 (who had recently visited China) was reported in Europe, specifically in Bordeaux (France). On February 26, 2020, the Brazilian Ministry of Health reported the first case of COVID-19 in South America. On March 11, with 118,000 cases reported in 114 countries and almost 4,300 dead people, the WHO declared that the outbreak of coronavirus disease caused by SARS-CoV2 was considered a pandemic and a “public emergency of international concern”, based on the diagnosis of a group of experts and guidelines (^{Pulcha Ugarte et al., 2020}; ^{WHO, 2020}).

Symptoms and preventive measures

COVID-19 occurs both in the elderly and in individuals with immunosuppression or chronic diseases such as diabetes, obesity, some types of cancer or chronic lung disease. In severe cases it can cause respiratory failure. Throughout the pandemic, it has been found that about 80 % of infected persons have mild symptoms or are asymptomatic. The most common clinical signs include those of a common cold: fever, cough and respiratory symptoms (dyspnea and other respiratory disturbances). Gastrointestinal symptoms including diarrhea have also been reported (^{Rodriguez-Morales et al., 2020}). In the most severe cases, the infection can cause bronchitis or pneumonia (either direct viral pneumonia or a secondary bacterial one), severe acute respiratory syndrome, renal failure and even death (^{Eliezer et al., 2020}).

In order to stop and prevent contagion, a series of measures were taken: isolation, temperature control with thermal cameras and digital thermometers, hand and home hygiene, 2-meter distance between people and the use of masks, among others. As the pandemic evolved, new ways of detection, diagnosis and prognosis of the disease have been incorporated (^{BBC, 2020}).