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COVID-19, from the perspective of year 2021, is a disease that results from infection with SARS-CoV-2, a coronavirus that apparently first infected human populations in Wuhan in the People's Republic of China towards the end of 2019. The disease was first described as an atypical ‘viral pneumonia’, and by January 2020, the infectious agent had been identified as a novel coronavirus. The origin of the infectious agent is uncertain: the current consensus is that it probably originated in bats, and that a novel mutated form of the virus, in which a spontaneous mutation had enabled the virus to infect humans, may have infected humans through interactions between live bats and people at a wild food market in Wuhan.
On January 22, 2020, The China National Health Commission reported the details of the first 17 deaths attributed to the new coronavirus: 13 males and 4 females, with a median age of 75 years.
By late January 2020, it was clear that the disease was being transmitted by person-to-person contact; the disease was rapidly spreading throughout Hubei Province, where Wuhan is located, and cases had been reported in several other countries, apparently attributable to travel from Wuhan. Accordingly, on January 30, the WHO declared the coronavirus outbreak a Global Public Health Emergency. 
By March 1st 2020, more than 3,000 deaths had been attributed to it, mostly in China. But the disease had already spread to several other countries, and on March 11th 2020 the WHO declared the coronavirus outbreak to be a pandemic. Soon virtually every country in the world was experiencing cases of the disease. By the end of March, the global death toll had exceeded 45,000.
In China, strong measures were taken in early 2020 to control the outbreak. Movement in and out of Hubei Province was controlled very strictly, and severe restrictions were placed on social contacts within the Province. This containment policy was combined with a rigorous policy of contact tracing and isolation, and enforcement of mask wearing and other precautions. These measures effectively suppressed the outbreak in China.
By December 2020, more than 66 million cases of Covid-19 had been reported globally; it seems likely that this is a considerable underestimate as many cases are asymptomatic, and many others with symptoms were never tested. The reported death toll exceeded 1.8 million.
The median incubation period for COVID-19 is estimated to be 4-5 days after exposure, but incubation periods of up to 14 days have been reported in exceptional cases. The spectrum of illness ranges from asymptomatic infection to severe pneumonia with acute respiratory distress syndrome and death. Of about 70,000 persons with COVID-19 in China, 81% cases were classed as mild, with no pneumonia or mild pneumonia, 14% as severe (defined as dyspnea, respiratory frequency ≥30 breaths/min, saturation of oxygen ≤93%, a ratio of arterial partial pressure of oxygen to fraction of inspired oxygen <300 mm Hg, and/or lung infiltrates >50% within 24 to 48 hours), and 5% as critical (respiratory failure, septic shock, and/or multiorgan dysfunction or failure).
Among more than 370,000 confirmed COVID-19 cases with reported symptoms in the USA, 70% of patients experienced fever, cough, or shortness of breath, 36% had muscle aches, and 34% reported headaches. Other reported symptoms included diarrhoea, dizziness, rhinorrhea, anosmia, dysgeusia, sore throat, abdominal pain, anorexia, and vomiting.
Most patients who die with covid-19 show notable signs of damage to the small blood vessels in the lung. There are extensive ‘microthrombi’ – clumps of red blood cells and platelets that may block the vessels. There are also extensive deposits of complement – proteins that contribute to the immune response to infection. In short, the lungs show the scars of a battle in which the defenders have caused as much damage as the invaders; the consequences of this appear to be that the lungs are left unable to provide enough oxygen to maintain life. It appears that those who die with covid-19 often do so because of ‘collateral damage’ caused by a hyper-aggressive response of their own immune systems to the infection.
There is also evidence that Covid-19 can lead to cardiac, dermatologic, hematological, hepatic,neurological, renal and other complications. The long-term consequences for survivors of Covid-19 are currently unknown. Persistent symptoms after recovery from acute Covid-19 have been described, and infection has been associated with a potentially severe inflammatory syndrome in children (multisystem inflammatory syndrome in children).
More than 1.8 million deaths had been attributed to Covid-19 by December 2020. There is considerable uncertainty about this number. Mortality from Covid-19 increases sharply with age, and is exacerbated by a range of comorbidities including obesity and diabetes. Accordingly some have noted that dying with Covid-19 does not always mean the same as dying from Covid-19. This factor might overestimate the rate of death due to Covid-19, conversely, in some settings there is considerable underdiagnosis of Covid-19 related death. There has also been concern that the measures taken to contain outbreaks of Covid-19 may have affected the rate of deaths from other causes. On the one hand, measures taken to limit transmission of Covid-19 are also likely, for example, to have been effective in limiting transmission of influenza, reducing the death rate from this disease. Conversely, effects on mental health of social isolation, and reduced access to health care may have increased deaths from diversity of other causes. Given these uncertainties, some believe that the most reliable estimate of deaths caused by Covid-19 come from measures of ‘excess deaths’ – the total number of recorded deaths in a region compared to historical data on the expected number of deaths.
The UK is one of the countries that has been hardest hit by the Covid-19 pandemic. In 2020 there were nearly 85,000 more deaths recorded in the UK than would be expected based on the average in the previous five years. By comparison, about 72,000 deaths were attributed to Covid (by mentions on death certificates), so it seems that there may have been some under-reporting of deaths due to Covid-19.  In some countries, a very much more extemsive under-reporting is apparent - and it has now been acknowledged that in Russia the true death toll of Covid-19 is at least three times higher than the reported number of deaths due to the infection. 
• People who are physically near (within 6 feet) a person with COVID-19 or have direct contact with that person are at greatest risk of infection.
• When people with COVID-19 cough, sneeze, sing, talk, or breathe they produce respiratory droplets. These droplets can range in size from larger droplets (some of which are visible) to smaller droplets. Small droplets can also form particles when they dry very quickly in the airstream.
• Infections occur mainly through exposure to respiratory droplets when a person is in close contact with someone who has COVID-19.
• Respiratory droplets cause infection when they are inhaled or deposited on mucous membranes, such as those that line the inside of the nose and mouth.
COVID-19 is mainly transmitted through respiratory droplets ("when an infected person coughs, sneezes or talks") from person to person between people who are in close contact with one another (i.e., within about 6 feet) .
The idea that droplets of virus-laden mucus are the primary mode of transmission guides the US CDC's advice to maintain at least a 6-foot distance. 
There has been ongoing concern about the extent to which covid-19 is transmitted by people who are infected with covid-19 but who are unaware of that because of the absence of any obvious symptoms. Early estimates were that as many as 80% of infections are asymptomatic, but these estimates now appear to be excessive, and have since been revised down to between 17% and 20% of people with infections. It remains unclear to what extent infected people with no symptoms can infect others. From viral culture studies, it seems that people can become infectious one to two days before the onset of symptoms and continue to be infectious up to seven days thereafter. It seems likely that symptomatic and pre-symptomatic transmission have a greater role in the spread of infection than transmission from truly asymptomatic people. However, asymptomatic and presymptomatic people make more social contacts than symptomatic people (most of whom are isolating). Hence to limit transmission of the disease, it is important that everyone, regardless of whether they are exhibiting symptoms, follows social distancing measures strictly and takes reasonable measures (mask wearing and hand washing). 
|Facts about Covid-19 as reported by the WHO to correct widely circulated misinformation.
• Studies show that hydroxychloroquine has no clinical benefits in treating Covid-19
• Vitamin and mineral supplements cannot cure Covid-19
• Prolonged use of medical masks* when properly worn, does not cause CO2 intoxication nor oxygen deficiency
• Spraying and introducing bleach or another disinfectant into your body will not protect you against Covid-19 and can be dangerous
• Drinking methanol, ethanol or bleach does not prevent or cure Covid-19 and can be extremely dangerous
• 5G mobile networks do not spread Covid-19
• The Covid-19 virus can spread in hot and humid climates
• Cold weather and snow cannot kill the Covid-19 virus
• Rinsing your nose with saline does NOT prevent Covid-19
• People of all ages can be infected by the Covid-19 virus
A great many myths have been propagated about Covid-19. Many of these have circulated on social media, including conspiracy theories about the origin of the virus, and claims that the pandemic is a hoax. Others have been circulated recklessly by various politicians, celebrities, and broadcasters. The WHO maintains a 'Mythbusters' site in an attempt to counter this misinformation with facts. 
- World Health Organization (WHO): Berlin, 13.01.2020Diagnostic detection of Wuhan coronavirus 2019 by real-time RT-PCR
- WHO: Coronavirus disease (COVID-19) pandemic
- NIH: Overview of Covid-19 updated December 17 2020
- BBC News: Covid: 2020 saw most excess deaths since World War Two
- The BMJ. Covid-19: Russia admits to understating deaths by more than two thirds.
- WHO: Scientific Brief: SARS-CoV-2 and Potential Airborne Transmission
- Centers for Disease Control and Prevention (CDC): Scientific Brief: SARS-CoV-2 and Potential Airborne Transmission
- Pollock AM, James Lancaster J (2020) Asymptomatic transmission of covid-19. BMJ 2020;371:m4851
- WHO: Coronavirus disease (COVID-19) advice for the public: Mythbusters